ATF is more stable, because it is single weight. Straight SAE20 weight oil is ideal really, in my experience, and as recommended by SU/Burlen:
http://sucarb.co.uk/technical-carbur...gs-chamber-oil
I found some single 20w oil for much less however (I think it was for a lawnmower, can't remember).
10w40 etc would change characteristics as the engine warmed up.