My dad has an '85 chevy van with a 350. He is very bad about maintenance, changes the oil maybe every 10,000 miles, but the engine runs like new with around 250,000 miles on it. The van has also received hard use, long trips and dirt/ranch roads often. He has always put chlorinated parafins in the oil and it seems to work. I don't know of any company that sells this stuff currently but it can be obtained for industrial use in 55 gallon drums.