GM recomends a 10w30 and strongly advises not to use a oil with that large of operating range. In 88 the owners manual point blank stated that they will void the warranty if anthing thicker the 10w 30 is used. That was pre sythetic but still that hot operating viscosity is WAY above the 30 the engine was spec for.
The engines after the mid 70's run tighter bearing cleaences and they will actully break down oils that have too thick of viscocity and demish their lubricating properties.
Also what brand of oil filter are you using? Many filters have a poor drain back valve and that will cause some start up rattle. Thats the motor running with no oil and it causes bad wear. The oiling in the area of the pump push rod is about the last thing in the line to get oil to begin with. It's crittical to use a guality oil filter.
I'm using A/C and Bosch filters. Bosch is badge engineered...it's a A/C filter. Other good filters are Wix, Hastings, Amsoil, Mobile1.
bad.....FRAM, anything from the oil change places, store brand.
1959 22' Caravanner
1988 R20 454 Suburban.