Trans oil for a "77"

Now please forgive me as I don't really want to open a can of worms here because I know everybody has their opinion and I have read a number of threads. My manual says to use S.A.E.20W, I know the manual is 45 years old and oils have come a long way since then. When I raced these bikes in the mid to late 70's I seem to remember always using Non Detergent oil in the transmission and the front forks, and we changed the oil often (after every event) and never had any problems.
Is this still a viable option?? I've been told just to use Spectro or Belray 80W gear oil and you don't have to change it that often?
The manual states S.A.E.20W and makes no mention of Non Detergent but I'm 99% sure we always used the non detergent. I do plan to race in some vintage hare scrambles. Does anybody use the old school oils or are most using the new gear oils???

Thanks, John

Torco MTFL 75W.

20w is a viscosity number
a ball falling through oil at a fixed temperature
modern oils stay in those viscosities
as stated above MTFL 75w is measured differently but is an old school 20w if that makes sense
don't get bogged down in brand names, pure synthetic can last longer depending on your riding
check it regular and change when dirty

2premo, so that being said, it would be ok to use a full synthetic multi grade motor oil (say 20-30)?? Or use a 80-90 grade gear oil? I do understand that a 20W motor oil is about the same as a 80W gear oil.
Thank you for the reply

in a word yes
I use synthetic oil for all my bikes
the modern gear oils are amazing