Originally Posted By: NO2
When is the last time anyone saw an oil failure because the oil was too thin and didn't protect the engine under extreme conditions? Probably never, because most people barely use 20% of their engine's capacity on a regular basis. Much more likely is to see sludge and varnish.
My son's roommate just seized her engine because she didn't change or check the oil for 4 years and it ran out. That is real world.
Actually I do think that some engines are being destroyed by thin oils but not because the oils are thin.
If the argument was only about thin vs thick viscometric behaviour, it would be hard to argue against thin. However, if you move from a 10W30 to a 5W30, all things being equal, thin is more volatile than thick (you have to lighten up the base oil mix to make the low temperature viscosity). If that extra volatility translates into more oil vapour being carried out of the crankcase, through the PCV, into the intake and burnt in the combustion chamber, then I think there is a significant risk that over time, the gummy deposits you get from burning oil can cause oil control rings to stick. Once you have a stuck oil ring, you start losing excessive amounts of oil from underneath the piston into the chamber. Once that happens, unless you want to do a complete rebuild (something that's not usually cost-effective on an older car), the engine is as good as knackered (as will be your cat if you live too long with high oil loss).
Yes, I know full well that a full synthetic 5W30 can have a lower volatility than a full mineral 10W30 and that thin doesn't automatically increase risk BUT if you have the right combination of circumstances (5W30, poor Group II, sloppy blending, low CCS to get FE, low tension rings, living in a cold winter climate, high blow-by, etc) then I reckon you can be heading for trouble!