Originally Posted By: SubieRubyRoo
A true solvent or a boutique oil that specifically references carbon removal like Valvoline Premium Blue Restore is the only thing that is really going to perform "cleaning". Several of the oil formulators here have reinforced that plain detergents do not actually "clean" so much other than maybe washing away some of the goop.
Kreen is probably one of the more highly regarded solvents on the board, Trav had a great 60+ page thread on it if you search. I used Kreen in my Odyssey and it took off a good bit but not all of the varnish under the valvecover. I'm sure a couple more rounds of it will get it looking as good as new, not that varnish alone really hurts anything.
MMO is tried and true but takes much longer, then you have Gumout AIO, Seafoam, and a couple others (Amsoil, Motor Mechanic, etc.)
What he said! ^^^^Restore is available at your local Cummins dealer. Valv Restore is specifically made to clean engines. I called the local Cummins in Alb and they will sell it to you a gallon if you wish. That way you don't have to purchase six gallons at a time.
Good luck!
Respectfully,
Pajero!