With all that, I don't see any reason that a 20w-40 or 20w-50
would not be a preferable choice to plain old SAE 20, 30, or 40 which are
all recommended in my 1987 manual.

operating weigh above recommendations can harm seals. thus using 20w-40 in
place of 20w doesn't do any good for lubing upon engine startup and can do some
harm when the engine is warm.

some people are under the mistaken impression that multi grade oils "thicken
up" as they warm up. they don't. they just thin out less as they warm up.