Gentlemen. Some time ago I broached the forum with the question--Does FP60 keep the underside of the intake valves clean? I received several responses with somewhat indefinite results. I recently had occasion to remove the cylinder heads from my 67 Mustang driver, this was with 20,000 miles after the initial valve clean-up. The engine is a 90 H.O. 5.0 with carburetion, some minor modifications and slightly massaged E7TE heads.( I replaced the head bolts with ARP studs and nuts ). Teflon valve seals are used with stock diameter valves. The oil is Mobil-1 10-30 with an approximate OCI of 7500 mi. FP60 is added at the recommended rate and the air/fuel ratio is slightly lean using a 390 CFM Holley. Were the intake valves clean? Yes. Best regards. John--Las Vegas.