Don't Worry about such advice...
I am using 10W-40 (ZIC) in 1990 Khyber and it's good grade over 20W-50. Also used 10w-30 Havoline Formula.
get engine flushed with sludge cleaner liquid, I got this from gawalmandi named WHIZ black bottle.
I shall also try same grade of others as well, Castrol Magnatec, Shell HX-7, ENOC etc.