My neighbors son (16) just bought himself his 1st car.... A 1978 Z28.Decient looking car 350 4V TH350 and probably 3.08 gears.He proudly showed me his new ride.He open the hood and I took a look. The engine appears origional and the origional owner said it has turned over twice so around 217,000 miles on it.He saidd the timing chain had been replaced as well as the other normal stuff. I pulled the dipstick and the oil was black.When I wiped the oil off the dip stick had a dark brown varnish stain on it....I took off the oil filler cap and ran my finger under the valve cover...It had a hard baked on grit all over them.Took a flashlight and could see a dark gritty feeling stain in the rocker arms.The engine pulls 17 in vac in gear at idle and 20 pounds of oil press at idle and 40 going down the road....Runs surprising decient and passed Calif smog. He asked me what oil to run. No real cold here , normally 40F to 95F temp range.... Would this be a cantidate for a high detergent type oil... maybe that Delo 15W-40 or is it best to not try to clean out any of the grit since it does seem to run well and just run something with less detergents ?? Any other ideas ?