What oil should i use??

[HotRod73]

as many of you know... i got a 1973 chevrolet C20 lwb with a 350/350 from my grandpa... it needs a tune up and i was wondering what oil you guys would suggest.... i was told not to change different types of oil or it will mess it up... but my grandpa has used all kinda oil in this thing (btw only 91k orignal miles and still runs gr8) i was told if i went from regular to synthetic i can wash out rings or something if hes never used synthetic before or something like that, is that true? i am trying to find a good answer here please help out on what type and brand etc i should use...