just put your premo unlead'd in if you really care, otherwise just run 91 reg.
I fyou having some trouble with detonation use the premo, (mostly noticable on hot days.
as for additives, if your after a higher octane just dissolve mothballs in 2l of petrol and chuck in your tank!!! (old tale the old, old guys talk about, works too appearantly!!!)
To be really sure what you should be running we need some specs stats of your engine, ignition, head, compression ratio, and cylinder pressures, also a realistic opinion about what your engine is doing when you load her up at slow speeds.
nato