It will be SAE 30Do the instructions say what grade you need?
I will price up 10/40 semi, see whats on the market.As above, 10/40 semi synthetic is just fine.
The mono grade 30 will tend to keep its viscosity better in the extreme temperatures that air cooled engines could potentially generate but in real life it’s not an issue.
Just change the oil regularly.....I do mine just as it’s starting to turn brown or yearly, depending on the hours used.
John