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Bad idea?
I have about 2l of 10/40 and 4l of 20/50 which was bought in error ( came up in a search for 15/40 and a tired fool only looked at price before clicking )
Both are semi synth, the 20/50 is reccommended for those yank tractor type things...

Wasn't there a chat a year or so back where someone had written to castrol and got a good reply back that might cover it .... Sorry don't recall the chat details to search properly for it and on phone so barely read the screen as it is :)
Just thought it might jig a memory with soneone

Reading around, there are very contradictory opinions. Different brands will have different additives which may impede each other, so you might be getting the best out of the resultant mix. Different viscosities from the same brand seems to be acceptable, but without any real idea of what viscosity the result will be. If the vehicle you intend to use it in has an acceptable range which covers both ends of what you have, the I'd go for it. That said I wouldn't put 20W/50 anywhere near my bike, even partially diluted. I'm not talking from a position of knowledge here, just gut instinct. I'd want the oil circulating the gear box and bores as soon as possible after turn-over, so I'd never put 20W/50 in the bike, compared to cars, the engines are in too high a state of tune and need that rapid circulation.

My tu'penneth.

That makes sense to me, thanks Daz

I think that the chat I referred to may be a sticky in technical I read loads of it last night but can't recall a thing

Check oilman's website, says its fine to mix different viscosities / grades as long as you don't mix different types. Loads a information on oilman's site.