Just to throw my 2 cents worth in on the Slick 50 issue:
I would recommend using a premium oil and forget about the additives. Premium oil has additives blended into a compatible package that meets all the needs of a healthy engine. I think synthetics have been the biggest improvement in automotive oil products in a long time.
An unhealthy engine is another story. Worn parts cannot be repaired/restored by any additive. One must realize that anything done to counter the effect of such an engine, like aftermarket additives, heavy oil, etc. is temporary, because an unhealthy engine is living on borrowed time.
YMMV
