My 98 GMC 1500 4x4 engine replaced with a used engine from A 2000 Chevy 3/4 ton van. Engine had 90 k on it and now has 125k. I change oil with Mobile -1 ( 5w-30 ) every 5k. Sometimes when its warmed up after a long drive 20 + miles the engine will quietly knock not very loud. The knock will go away after start up...I am considering going to a heavier oil. Maybe 15-40 Mobile-1. What say you?

 

That or an oil additive IMO.

 

I already use Lucas Synthetic Oil Stableizer every oil change...

 

Hey man. I use 15/40 in my 89 305. I cant say I use it for a knock I just use it because in my opinion it's just better oil. The pump has no issues with it and I've been running it for 5 years. As for the engine knock... Who knows man if your not experiencing lower than normal oil pressure I doubt there's any clearances that have changed. Maybe your just hearing the natural rotational noise of the engine and attributing to a knock?