GM-GMC: oil change, oil pressure gauge, piston slap

Question
89 gmc c2500 5.7 tbi had oil changed at well known franchise place.they also recomended flush out, let run 15 min with stuff he put in oil.then drained,added oil and was about 1/2 mile away when motor started knocking real loud like rod bearing and smoking from exhaust kept stalling,drove back to place but were no help,oil pressure gauge reads 60-80,if i rev up a little the knock goes away.thanx for help with this.

Answer
This is not the first time ive heard of this problem.. you guess is probbally right. I have repaired engines with rod knock, crank bearing knock and piston slap after these treatments. these products can clean so much carbon and varnish fronm the engine that it brings the real internal problems to the surface.  really sorry ! some times thicker oil might help.