What we are really concerned about here is oxidation. Wear is rarely if ever going to be factor at the change intervals you are using and provided you don't run too lower vis.

Your wear metal numbers back that up.

Total acid number and how it trends upwards and Total base number and how it trends downward are used as Industry standard indicators of oxidation.

In fact some fleet operators use the point at which the TAN and TBN curves cross as the life of the oil. e.g. once your reserve of detergents and soap is used up, TBN will drop rapidly as oil becomes more acidic and oxidation rapidly occurs.

As I am sure you know, oxidation is an exponential increase so once it starts, the acidity of the bulk oil (plus insolubles) is a key indicator.

Hence, rapid rise in TAN and depletion of TBN = onset of oxidation. Vis at 100C will now also start to rise.

The above is on an over simplification as some components in your oil are intentionally consumed and degrade in order to function but 99% of Industry standard oxidation tests have some delta TAN or absolute TAN limit to predict oil life.

There is a lot of talk here about Euro Castrol Syntec. The Euro synthetics in general have to meet ACEA specs A1/B1 and A5/B5 which involve more stringent oxidation tests (specifically VW turbo diesel which is an oil killer).

Look into the new Mobil 1 Extended performance...it meets ALL the claims of the Euro Syntec. and can be picked up reasonably at Wally world in gallons.

If someone wants to discuss more with me, drop me a line.

But always remember, oil is cheap !

DISCLAIMER : In case you are concerned, I do not work for any major oil company or am I an independent oil dealer (Amsoil). I just stayed at a Holiday Inn Express last night.

2000 Santorin S4 Slush box
710Ns, APR 1's and 0's and AudiSport Catback
18" RS4 DTM reps w 235/40/18s
ICE LINK Plus
RS4 grille and splunge valve
(Yeah, I know I am a nOOb, thanks
but I got VAG-COM )