MV never listed a quantity or weight for the fork oil on the old singles.
From experience around 125cc seems to work OK. You might fins 20 weight a bit too stiff, but then again it all depends on your riding style etc. I use 10 weight on most of mine and it seems to work OK, however if the forks are a bit worn a slightly thicker oil might be better. A bit of experimentation might be in order!
Thanks for the fork oil info.
The forks are the ones that I got from MV Spares, they were in very good shape. I only had to clean them before new seals.
I will try the 10 weight oil first.