use ATF!

best thing i use for engine longevity and keeping it clean inside is substitute a qt. of ATF in place of 1 qt of oil when ever you change your oil. lets face it, the auto trans sees far higher operating temps, and much greater working pressures than an engine ever will! the ATf has an expreme pressure package and detergent package that rivals the best additives! and it costs way less than those snake oil additives!
my 84 jeep with a straight 6 has 300,000 miles on it , and i hav`nt had the valve cover off!( not even to fix that damn leak!)been doing this in all my vehicles for 20 yrs with NO engine failures at all!