Ive got over 900000 miles on my Detriot 60 series engine.oil samples are fine,and oil change intervals are every 20000 miles.In the beginning,I've gone as high as 30000 miles and still had good samples,but i feel more compfortable at 20000,thats when it finally has 1 gallon missing.If you talk to any oil experts,you will stop putting additives in your oil,they say your just waisting money.In my cars I use a synthetic oil,as they sit around more than get driven.A truck I had in 1993,I used Amsoil synthetic.Truck went 100000 before had to actually change the oil.It did have the Amsoil filter system,and change filters every 15000 miles,with an oil sample.Only changed the oil at 100000 cause was nervous going any longer.Whats better about synthetic oil,it doesnt break down with the heat of the engine,but the additives you put in the oils ,are just witch remedys.If it makes you feel better great,but as I was told,just waisting your money.By the way,Ive been useing Rotella oil in every truck I've ever owned,except the 1993,when I used the synthetic.Why am I not useing it now,I usr ro be a dealer,and it was very cheap to buy,and now I live in an apartment ,so storage is a problem.