mixing syth oils
 

is it a totally bad idea or is it ok?

I currently have shell adv ultra 4 in the bike (or so they claim :cool: )
I have some spare castrol superbike 4

Both are fully synthetic oils.

My oil is a bit low and I don't have time to change it for a couple of weeks: if I do a top up, mixing the synths, will it turn to black sludge? :bouncy:

No, it wouldn't turn to sludge......it'd be OK! And contarary to popular belief you can mix sythetic with mineral......:o......you'd just end up with a semi.....

but is it true that semi-synths are just as good as synths for road bikes??? :)


Now where did I read that? hmmm must have been in mcn I think.

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]