Here it is, straight from www.castrolauto.com:<p>"Under the high temperature and high shear conditions found in today's engines, low viscosity oils provide a thinner cushion of oil than high viscosity oils. As a result, engines lubricated with a low viscosity oil can be more susceptible to metal-to-metal contact. This is especially true in hot climates, during the summer and under high load or severe service conditions. Additionally, low viscosity conventional oils contain "light" compounds that volatilize or evaporate at high temperatures. This can result in the formation of harmful deposits and an engine running low on oil. "<p>Now, I don't take anyone's claims at face value, but this is exactly what common sense has been telling me for years. I have no oil loss, no hydraulic lifter clatter, and no apparent bearing wear (and yes, I have analyzed my old oil for various metals) using Mobil 1 15W-50. And I don't baby my engines, either! From being on this board for a year and a half, I'm willing to guess those discussions about 1/4 mile ETs and road racing / autocrossing would put most of us into the "severe duty" category!<p>I know when I finally start to autocross, I'll be glad to have a viscous, hard-to-shear oil layer on every part of my valvetrain and turbo!

Big O,<p>I use Mobil 1 15W-50 all year around, here in Seattle. I did run 10W-30 once in the winter in my 90Q, but my mechanic had a fit. He doesn't feel that the 10W-30 offers enough protection when the engine is at operating temp. He also feels that the 15W-50 synthetic is sufficiently pumpable in the lower temps at startup. The 90Q is very close to 100K, and there is no evidence of oil related wear, and actually, the engine is running better than ever. I switched the A4 to Mobil 1 15W-50 at 7500 miles. How many miles do you run the Mobil 1 in your car? I run it 6K w/ 3K filter on the 90Q, and 7.5K w/3.75K filter on the A4. I've actually been considering going 9K on the 90Q, while maintaining the 3K filter intervals. The Mobil 1 holds up well, so I see no reason why it wouldn't be fine. Like you, I don't baby my engines either. Anybody here who is doing the track events and what not should really consider using the higher viscosity synthetics.<p>Charles<br>

Castrol Syntec 5W-50? Is there something I am missing? It seemed to me that there are a lot more people using Mobil. Why?

Mobil has as much or more experience with synthetics as just about anyone (Red Line & Amsoil may come close in years, but not R&D $$ and experience). I have seen enough technical data on Mobil 1, and enough first hand stories of how Mobil 1 protected engines in various failure modes, that I'm confident in Mobil 1, but not so with Syntec (Not saying it's bad, just that my personal choice is to not use it). It's either Mobil 1 or Red Line for me (and perhaps Amsoil). Amsoil (and Red Line, I believe) use a different base stock than Mobil 1. As fine as Mobil 1's base stock is, even a Mobil tech rep admitted to my Porsche tech that the Amsoil base stock has better properties for extended drain intervals (to fight condensation, corrosion, acidity, etc.). This pertains to the extended drain interval if you're using oil analysis to determine when to change, not the ability of the oil to protect the engine (these 3 all do well there).

Is Mobil 1 5W-50 a high viscosity oil? Is it better than 15W-50? I live in Vancouver.

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I use it in all my vehicles, from my 5.0 Mustang to my ZX-11, but especially in my Turbo and my 944S 2.5L (high-stress high-RPM four cylinders).<p>I go 10k to 12k in the Porsche, monitoring the oil condition by simply eyeing the color and viscosity. When it darkens, I change it. That's been keeping the lil' 944 clean and healthy, with *no* oil usage between changes except maybe at the very end (another indication it's time to change, so I usually just change it rather than topping off). It never gets below the bottom of the fill indicator.<p>The 1.8T gets it every 5k miles, but last time we went in for the pesky airbag light, so I had them change it at 3k, thus the next interval will be 7k (now at 25k, had it at 23k, will go again at 30k). I bring my own 5 Quart jug which Schaumburg has no problem with, and they even asked me how full I wanted it (where on the full marking).<p>Here's my evidence for why I'm using this: Take any refined oil (5W-30 for all I care) and 15W-50 Mobil 1, and heat them both in teo pans (yes, I know, it's like the commercial). Feel the oil film and see for yourself how much thinner the non-sytnthetic is at high temperature, and watch it discolor at high temperature.<p>Than take the same two oils in their bottles, put them in the freezer, let 'em cool way down and try pouring them out. Betcha the Synthetic flows better at really, really low temperatures; it won't "gob" up like even the 5W regular oil does.<p>That's all I needed to see. Plus, my bike dealer's mechanic, who races bikes, came up to me and said "hey, great, finally someone who realizes what oil to use in a high-rpm bike" when I refused to use the then-recommended special "bike" oil at $6/quart. That was 1990. Now, the manufacturers actually recommend Synthetic. So does Porsche.

The correct weight of oil depends on the ambient temperatures at which you'll be driving and the car. The A4 owner's manual quite clearly spells out what weights to use under what temperatures (and also takes into account the energy efficiency rating).<p>Tom<br>1.8TQMS Silver/Ecru

In an ideal world, oil would not change viscosity with temperature, but it does. So, to make it act a little "nicer" and use it all-year round, manufacturers add viscosity modifiers (additives) to make it act like a thinner oil at low temps and thicker oil at high temps. This is not to say that it is *thinner* at low temps; it just doesn't thicken as much as a regular oil would (try watching straight 30 weight go from zero to 200 degrees and see sludge turn into water). It acts as a viscosity "buffer", if you will.<p>Synthetic oil acts a little more reasonable that dino oil in that it doesn't have sudden dramatic viscosity swings, so it needs less additives to begin with. But to get a regular oil to act like a 5 in winter (reasonably thin) and a 50 at high temp (fairly thick) takes a LOT of addititves, and these additives tend to break down faster than the oil does. So a 5W-50 would be prohibitively "modified" and may not last too long.<p>Synthetic, in other words, is much closer to a "perfect" oil to begin with (there aren't infinite lengths of molecules all acting differently as there are in dino oil since synthetic is created artificially, so every molecule is exactly like every other, accounting for the more controlled viscosity changes), so it takes very little to make it act like a 5 in cold and 50 in heat, and thus there are much less additives to break down and foul up both the engine and the oil's behaviour.<p>And I'm still willing to bet that if my dad had 15W-50 in his lawn tractor BOTH times he ran it low on oil and STILL finished the yard, it would not have seized up with nothing but black sludge in the 20HP Kohler twin engine! That's a $800 engine swap just because dad keeps insisting on following his old manual that does not mention synthetic, so he won't use it . . . . aaarrrgggghhhh!<p>Big synthetic O!

The manual recommends 10W-30 in the A4 1.8T, but should I be using the 15W-50, really? I figure that synthetic 10W-30 should be good enough protection for my turbo, even in the summer.<p>Does anyone think this is dangerous?<p>Darryl<br>98.5 1.8t qms

Whether an oil is synthetic or not will not change its viscosity characteristics as a function of normal operating range. A 15W-50 normal and synthetic oil will flow the same at 0 and 200F - that's what the oil weights mean. Now, I do agree that the synthetic won't break down, out gas, coke-up as much... My opinion.<p>Tom<br>1.8TQMS Silver/Ecru

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Now, you're right, I know the weights are supposed to reflect the viscosity, so what exactly indicates the better flow of the synthetic at low temperatures?<p>I'm entirely in agreement with you, though, I'm just wondering *where* and *how* the viscosity index is measured. At a particular temperature? Across all temperatures? If so, what explains the flow differences between synthetic 15W and regular 15W at extremely low temps?<p>My understanding was that the different molecules in the dino oil act differently under extreme temperatures, including the paraffins (waxes) which make the oil "clumpy". Since dino oil is "refined", i.e. it's the best attempt at selecting a particular molecular property/structure from a wild (naturally occuring) mix of molecular lengths, it invariably contains some residual paraffins. Synthetic is "made to order" (from homogeneous substances), it doesn't have these residuals.<p>Hey, where's that Organic Chemistry book . . ?!?

We can discuss this until we're blue in the face, but if a sudden mishap drains three of the four and a half quarts out of my car, if I miss a shift and wrap the crankshaft into bearing-scoring territory, or if my wife overheats the engine before noticing something pierced the radiator, I'll take the oil that provides a Kinematic Viscosity rating of 18.0 cSt (15W-50) over the 9.7-10.0 of the lower weights at 100 degrees C every time! <p>Please note, this is NOT intended to say you're wrong, just that I like to have that extra layer of protection. There is a reason they recommend it for "high performance engines" and "racing duty".<p>The normal loss of pumpability experienced with paraffin-containing refined oils is not as prevalent with synthetics, so it pumps better than most normal 5W-30s. For that matter, the 15W-50 synth pumps better at -25 than the 0W-30 at -40.<p>Granted, if I were to leave the A4 sitting at the arctic circle in -90 degrees, I'd use the thinnest oil I can find. But ours sits inside and isn't exposed to the winter cold any longer than three hours at a time, so the risk of the block ever getting to under -25 degrees is miniscule. But an hour into a nice, hard run using the G-tech and/or racing around a road course, and the oil and turbo will be just about as hot as they are on an average summer day. At least that's what my gauge says.<p>Sorry if I seem so anal about this; I just recently got into this discussion with my father after he blew up his tractor and then almost seized his Audi because he runs them VERY low on oil (he uses the oil light and/or lifter tick as an oil change indicator) and won't listen to my advice on giving up on the straight 30W oil ! :-0

I know it's manufacturer blather, but it does *not* compare it to the cheaper product, just to the *other* viscosities, so they have no incentive to sell you the heavier weight over the others.<p>http://www.mobil1.com/oneracing/about/products/engineoil/15w_50/index.html<p>Note that it states "less oil consumption", which is one of the reasons I have kept my faith! That one I could measure and prove!<ul><li><a href="http://www.mobil1.com/oneracing/about/products/engineoil/15w_50/index.html">Mobil One Spec Sheet for Synthetics - Good Stuff !</a></li></ul>

I think the following is correct... The W number is the viscosity index for the oil at 0F (-32C) and the 2nd number is the viscosity (weight) index at 212F (100C). Each weight of viscosity index represents a range of actual viscosity. Therefore a 15W-50 oil is actual THINNER (lower number) at 0F than at 212F. You want it to be thinner (I think) because you don't need the protection (cooler temps) but you do need to get the oil to all of the places in the engine as quickly and uniformly as possible. At higher temps, you need the protection that thicker viscosity brings. Now at extremely low temps (less than -10F) synthetic has a BIG advantage - its pour point is much lower than typical nonsynthetic. The data I've seen (can't recall the website) shows that most synthetics pour point is below -50F while for many normal oils its no lower than -10F. That's where the synthetic nature of it helps a lot. Hope this helps.<p>Tom<br>1.8TQMS Silver/Ecru<p>

At least according to my dealer, the information about oil weight in the Audi manual is incorrect. The car needs heavier oil than the manual indicates. I haven't actually called Audi myself to confirm this, but he says that he did. Why doesn't Audi reprint the manual? Who knows?

The last oil I put into my qT was Mobil 1 5W50, widely available even from Kmart and Target here downunder!!<p>Graham

They may not sell it in your neck of the woods, but they do make it as I have it.<p>Graham

You bring up some good points which I was thinking of too, earlier when I checked here, but had no time to respond. <p>I agree with the use of lower viscosity oils in severe climates. If you are regularly starting your car at sub zero temperatures, maybe a 5W-30, or 10W-30 would be the best way to go. However, no matter how cold it is outside, once the engine is warmed up, it's operating at normal temperatures, just like you said. For those running the lighter oil, I think it's risky running the engine hard when it is hot and has the lower viscosity oil in it. So, there are some tradeoffs. Like you said though, the 15W-50 synthetic is more pumpable than a lighter conventional oil, and so it's not a problem. This is why my mechanic felt it was better to use this weight. It offers much more protection at extreme operating temperatures, while offering better startup characteristics than conventional oils. BTW, he doesn't use anything lighter than 15W-40 in the cars he services, as far as I know. He specializes in Porsche, Mercedes, BMW, Audi, and VW.<p>Just a side not about how I drive. Always floored, almost never shift before 4K (except when engine is cold), Regularly run to 5K or 6K RPM (believe me, regularly = a LOT), downshifts while braking, and high engine temps in traffic. I'm going to stick with what I feel will be the best protection for the way I drive. I don't think the 10W-30 would offer the protection I need under these conditions. <p>Charles<p>PS: Ambient temperature does not equal engine temperature! :-) Weigh your requirements.

I think Mobil 1 also makes a 10W-40, but that is not available here either. If it were, it may be a compromise I would consider making for winter use. Oh well!<p>Charles<br>

rany info on Vollsnthese?

so get Redline 10w40

I use the redline MTL in the gearbox, but using Redline in the engine is just too much of a hassle. Mobil 1 is available on just about every corner, so I can get it whenever. Getting the Redline for the gearbox is enough of a pain in the butt. If I had to go through that every time I changed the engine oil, I'd go even more crazy than I already am. Besides, there isn't really any reason to use the 10W-40. 15W-50 is fine. Seattle winters aren't very cold.<p>Charles<br>