That info is in the "Instruction Book", aka owner manual.
Normal temps 50-90F the recommendation is straight 40w. They do say 20-40 multi-grade is acceptable.
Today's reality? 20-50. I use Valvoline 'racing' oil because it has the extra zinc and holds up to thermal breakdown much better than 'regular' oil. It's what I do; but this being an oil thread you'll get fifty different opinions.
Transmission the manual essentially says use the same oil as the engine. And, it was typical to use a similar oil in the primary. I used some leftover 10-40 in my gearbox. In the primary, I use Type F automatic transmission fluid. It is what is recommended for my Bullet and seems to work fine in the (basically identical) clutch of the Interceptor. Bonus: it's a different fluid with a unique color, so you can know what is leaking on your floor a bit easier!