There were oil marks on the dipstick above the 'min' level, but none of the marks were very clear. Will try leaving the dipstick in for longer to see if that helps.

It's always best to read the dipstick in a cold/cool engine, on a level surface - straight after switching off will never give a true reading, and the oil is less visible too !

I've used the lowest cold viscosity with the rated hot viscosity - that gets oil around the engine fastest when it's cold and warming up - which is when most engine wear occurs

Thats the story, sure.

Might even be true, sometimes, but the (rather few) actual studies I've seen of the effect of oil viscosity on engine wear, tend to show it goes up,a little, as the viscosity goes down.

Achieving these hot viscosities with a low base viscosity requires a lot of viscosity index improver and these can tend to clog oil control rings with their degradation products.

This doesn't matter much to me, since something other than engine wear (usually THE MAN in some form, oft abetted by my mechanical incompetence) has tended to kill my cars, but its not a big surprise if the studies differ from the advertising copy..

Edited by edlithgow on 02/05/2024 at 05:39

For 3 decades now, I've used the lowest cold viscosity with the rated hot viscosity - that gets oil around the engine fastest when it's cold and warming up - which is when most engine wear occurs - I'm presently using 0W-30 in my 3.0 TDi VW which is rated for 5W-30 - but importantly I do ensure it meets the correct VW specification.

My late lamented Skywing had very little carbon on the piston crowns and in the ring and oil control grooves, despite being ancient, and running a bit rich by the plug appearance.

This could be due to sometimes using liquid water decoking in the running engine ( though I believe this may have caused spalling damage to the cylinder walls, and wouldnt do it again. I would use steam instead, which might have other Smokey Yunik stylee advantages)

It could also be due to the use of relatively high viscosity oil with a relatively high detergent/dispersant content, a relatively low viscosity index improver content, and a relatively low volatility.(I used a 1 to 1 mix of CPC SAE 40 and Mobil Delvac MX 15W40)

Low viscosity oils tend to be more volatile, as per the NOAK volatility test, and such volatiles are more likely to be carried over entrained in blowby gases via the crankcase ventilation system and burnt.

Low viscosity oils are probably also more likely to pass the piston rings and be burnt directly. Both tendencies will be aggravated by higher compression ratios and turbo-boosting.

Discussed here, with a technical contribution by Sonofjoe, whos also been on here.

bobistheoilguy.com/forums/threads/how-common-are-s...2

He also suggests (I think plausibly but speculatively) that more stable synthetic oils might actually produce more combustion carbon deposits.

The Skywing didn't use much oil.

The Skywing's engine also got a lot cleaner generally (outside the combustion chamber) during the 10 years or so of my ownership. (as seen under the valve cover and in the sump) loosing the extensive varnish sheets it had when I bought it.

This cant be due to steam cleaning or lower volatility (which would only affect the combustion chamber) and probably wasn't down to lower viscosity index improver content either. My guess would be that the relatively high solvency of the Group 1 SAE 40, combined with the dispersancy/detergency of the Delvac, shifted the varnish, while my mileage was too low for extensive oxidation of the inherently less stable non-synthetic oil.

All a bit speculative, based on anecdotal evidence, as is yours.

But I prefer my anecdote. No creative input from VW, for one thing.

Edited by edlithgow on 17/05/2024 at 07:23