Oo you know this one causes arguments. 20/50 mineral oil should be used for old motors Cue the debate.
10/40 mineral here... Although I have heard that freshly squeezed fish oil really perks the engine up after a long rest.
Straight 30, and petrol, the petrol was just a by product of some weirdness going on in my engine though.
We use 15/40, and I've no idea if it is the best one, but it's what the guy who had the bus before us ran it on, and he seemed to know what he was doing.