| Moozaad,
In theory, you should be better protected with a fully synthetic oil Vs a semi, purely cos the FS oils have been developed purely with a purpose in mind (be it sheer stability, anti foaming, free-er revving etc, etc). If there's any shortfalls in the composite mixutre, these can be irradicated via technology and additives. There are certain limitaitons with semi's cos you are relying somewhat on the constraints of a mineral element in the finished product.
Having said that I've had two big end bearing failures on two 748's using the same fully synth oil. Some say it's the oil and others don't.
Still wouldn't make me consider putting a mineral or semi in my £11k piece of technology (just rip it apart more often and replace shells).
Mixing viscosities will have the effect of moving the thicker of the two oils more towards the viscosity of the thinner one (i.e. putting 10w/40 into a 3/4 sump of 15w50 would result in a mixture somewhere in beween the two). The only way it should end up as black sludge is if you leave it in there for a few years and have crap piston rings.
Guido (ex-Castrol employee)
[Edited on 12-8-2004 by Guido]  |