Hi Dave
This is annoying, I used to have one of these but didn’t keep a copy of the engine manual. SAE 40 is definitely too thick. From memory I’m pretty sure for UK climate it’s SAE20, for warmer countries SAE30. But I could be wrong. These are heavy engines to turn over, especially when cold in winter, at one time I put too thick oil in (30 I think) and could hardly turn it over in winter.
Peter