John Wilkinson
10-22-1999, 02:37 PM
Hi all,
I just had an enlightening discussion with some MIT engineers (graduates working at MIT that is, not students) about motor oil. We all own 1.8T engines (A4's and Passats), and we were raving about them, and were all in general agreement about what engine oil to use (synthetic, 0W-, 5W-, or 10W-30 for Boston area). The interesting part of the discussion, however, revolved around some things about motor oil relating to warm-up and viscosity grade selection, that I (as a physicist rather than an ME) never knew before.
1st myth shattered: the grade before the 'W' in xW-y refers to the oil's viscosity at 0 C (32 F). I'd always believed this, and I've read it numerous places on the web while researching oil, but it's not true! The SAE has a specification for cold-cranking viscosity at a certain temperature, which varies for the different winter grades. 15W-anything, for example, must have a cold-cranking viscosity of less than 33 Poise at -15 C (5 F). 10W has the same spec, but measured at -20 C (-4 F), 5W is measured at -25 C (-13 F), and 0W is at -30 C (-22 F). (IIRC, 0W is allowed a higher viscosity at -30 C due to the difficulty of making *any* oil flow at -30!)
What this means, is that if 15W is OK down to 5 F, as indicated in the 1.8T owner's manual, 10W is just as good at -4, 5W at -13, etc. It also means that the claim that synthetic 15W-50 (Mobil 1 or other) is better than conventional 5W-30 at low temperatures is incorrect! The 15W-50 may pump better at *really* low temperatures (like -55), but at 5 degrees (the lowest VW recommends for 15W), the 15W-50 synthetic performs exactly like any 15W conventional! This is probably why Mobil and others are quick to point out that their 15W-50 pumps/pours at temperatures as low as conventional 5W-30, but stop short of recommending that it be used in place of a lower winter grade oil. Mobil merely states that, "Mobil 1 Formula 15W-50 can be used where SAE 15W-40, 15W-50, 20W-50, 15W, 20W, 20, 30, 40, or 50 are required."
2nd interesting tidbit: The important thing to do right after startup, when the engine is still warming, is to avoid high engine-speeds, but not necessarily high-load. The reason is this (and again, this applies equally to synthetic and conventional oils of similar viscosity grade): before the oil reaches normal operating temperature (~212 F), where viscosity is about 10 cSt (for an SAE 30 oil), the viscosity is *much* higher -- ~60 cSt for 5W- or 10W-30 and ~120 cSt for 5W- or 15W-50 at 40 C (104 F)!
Assuming that you've waited at least 10 sec for the oil to be pumped to wherever it needs to go, metal to metal contact isn't your worry, even under heavy loads (remember the viscosity is very high at this temp, and high viscosity promotes thick oil films between moving parts). The problem is at high engine speeds (and also within the bearing of the turbo) where high-speeds will cause the viscous oil to thicken its film to reduce it's internal friction.
If your pistons have 'x' in. of clearance between them and the cylinder walls, and you're revving the engine at such a speed that the viscous oil wants to form a film of thickness '2x' in order to minimize friction (twice the film thickness allowing half the shear stress), then the oil may actually pool on _one_side_ of the piston and push it up against the bare cylinder wall on the other side! Rev too high when the oil is too viscous and you may actually push all the oil out of the cylinder. (The cold oil, subjected to extreme shear rates, may literally turn to silly-putty-like consistency, and be stripped from the moving engine parts.) _Then_ you'll get metal-to-metal contact.
Fortunately, this is an extreme case, and doesn't happen often. It also happens less often with energy conserving oils, because their friction reducing properties mean that the film doesn't have to be as thick in order to lower shear stress to acceptable levels. _But_ this does mean that 5W- or 15W-50 users will have to wait longer for their engines to warm-up than do users of energy conserving xW-30. The tighter tolerances in modern engines make them more prone to the problem described above, and are one of the reasons that fewer engine makers recommend the higher viscosity-grade oils.
Just thought I'd pass along the info, and I hope you all enjoyed it!
-John
P.S. I'm working from memory here, about a conversation had earlier in the day, and I have no handy references to check, so please forgive any minor inaccuracies.
I just had an enlightening discussion with some MIT engineers (graduates working at MIT that is, not students) about motor oil. We all own 1.8T engines (A4's and Passats), and we were raving about them, and were all in general agreement about what engine oil to use (synthetic, 0W-, 5W-, or 10W-30 for Boston area). The interesting part of the discussion, however, revolved around some things about motor oil relating to warm-up and viscosity grade selection, that I (as a physicist rather than an ME) never knew before.
1st myth shattered: the grade before the 'W' in xW-y refers to the oil's viscosity at 0 C (32 F). I'd always believed this, and I've read it numerous places on the web while researching oil, but it's not true! The SAE has a specification for cold-cranking viscosity at a certain temperature, which varies for the different winter grades. 15W-anything, for example, must have a cold-cranking viscosity of less than 33 Poise at -15 C (5 F). 10W has the same spec, but measured at -20 C (-4 F), 5W is measured at -25 C (-13 F), and 0W is at -30 C (-22 F). (IIRC, 0W is allowed a higher viscosity at -30 C due to the difficulty of making *any* oil flow at -30!)
What this means, is that if 15W is OK down to 5 F, as indicated in the 1.8T owner's manual, 10W is just as good at -4, 5W at -13, etc. It also means that the claim that synthetic 15W-50 (Mobil 1 or other) is better than conventional 5W-30 at low temperatures is incorrect! The 15W-50 may pump better at *really* low temperatures (like -55), but at 5 degrees (the lowest VW recommends for 15W), the 15W-50 synthetic performs exactly like any 15W conventional! This is probably why Mobil and others are quick to point out that their 15W-50 pumps/pours at temperatures as low as conventional 5W-30, but stop short of recommending that it be used in place of a lower winter grade oil. Mobil merely states that, "Mobil 1 Formula 15W-50 can be used where SAE 15W-40, 15W-50, 20W-50, 15W, 20W, 20, 30, 40, or 50 are required."
2nd interesting tidbit: The important thing to do right after startup, when the engine is still warming, is to avoid high engine-speeds, but not necessarily high-load. The reason is this (and again, this applies equally to synthetic and conventional oils of similar viscosity grade): before the oil reaches normal operating temperature (~212 F), where viscosity is about 10 cSt (for an SAE 30 oil), the viscosity is *much* higher -- ~60 cSt for 5W- or 10W-30 and ~120 cSt for 5W- or 15W-50 at 40 C (104 F)!
Assuming that you've waited at least 10 sec for the oil to be pumped to wherever it needs to go, metal to metal contact isn't your worry, even under heavy loads (remember the viscosity is very high at this temp, and high viscosity promotes thick oil films between moving parts). The problem is at high engine speeds (and also within the bearing of the turbo) where high-speeds will cause the viscous oil to thicken its film to reduce it's internal friction.
If your pistons have 'x' in. of clearance between them and the cylinder walls, and you're revving the engine at such a speed that the viscous oil wants to form a film of thickness '2x' in order to minimize friction (twice the film thickness allowing half the shear stress), then the oil may actually pool on _one_side_ of the piston and push it up against the bare cylinder wall on the other side! Rev too high when the oil is too viscous and you may actually push all the oil out of the cylinder. (The cold oil, subjected to extreme shear rates, may literally turn to silly-putty-like consistency, and be stripped from the moving engine parts.) _Then_ you'll get metal-to-metal contact.
Fortunately, this is an extreme case, and doesn't happen often. It also happens less often with energy conserving oils, because their friction reducing properties mean that the film doesn't have to be as thick in order to lower shear stress to acceptable levels. _But_ this does mean that 5W- or 15W-50 users will have to wait longer for their engines to warm-up than do users of energy conserving xW-30. The tighter tolerances in modern engines make them more prone to the problem described above, and are one of the reasons that fewer engine makers recommend the higher viscosity-grade oils.
Just thought I'd pass along the info, and I hope you all enjoyed it!
-John
P.S. I'm working from memory here, about a conversation had earlier in the day, and I have no handy references to check, so please forgive any minor inaccuracies.