I have an 89 K_2500, 350CID engine, overhauled at approx. 100K miles, now has approx. 140K miles. Has started smoking when idling on a hillside at a traffic signal, etc. I suspect valve seals need to be replaced. Heads were also rebuilt by a "mediocre" machine shop at 100K miles. My question is: Should I replace the valve seals with what the OEM used or is there something better? I "think" I remember reading in the shop manual that different types of seals are used on the intake valves vs. the exhaust.........does that sound correct? Comments please.