20w50 is ideal in any mini engine, as the gearbox runs in the sump.
look at any modern car, and you'll see that they use a thicker oil (like ep80w90) in the gearbox, and a 5w30 in the engine.
now you can't fill a mini engine with ep80w90 gear oil, its just too thick for the oil pump to cope, but will make your gearbox last ages. so the next best thing is 20w50, its thick enough for the gearbox to feel slick without premature wear, and thin enough to pass round the engine's oil galleries and bearings. I run valvoline racing 20w50 (roughly £19.00 for 5 litre) in my mpi, and millers CTV 20w50 in the 1380. the ctv is about £35 for 5 litres, but is specifically designed for gearbox in sump applications like the mini, something to do with the chemical compund, makes it suitable for gearboxes and engines.
the mpi used to run on 10w40, which i think is too thin for any mini, purely because it gives the gearbox very little protection. the gearchange used to be quite notchy and stiff, i put the valvoline 20w50 in, the engine is much smoother running, the gearbox is quieter, the gearchange is super slick now its like a new gearbox. and the annoying leak that nobody could cure has gone!
20w50 is the future!
With every post, I feel better and better about my oil leak - hopefully I'll clear it up shortly - thanks!
The Valvoline "racing" oil you use - is that the VR1 formula in 20W-50 grade?