I constantly run a quart of MMO at all times, put in about 4qt of Rotella T6 and 1qt of Marvel. According to Marvel themselves it should cut the 5w40 to roughly that of 5w30 in that amount (believe it also says so on the back of the bottle how to calculate it). My initial reason for doing so was to help clean out some cooked oil in my valvetrain and generally clean out the system, and it's worked pretty well for it but it did take a 1-2 full OCI's to see any change.
Some people also use Gumout Multi-System in the oil but I've yet to do so. I believe it's only something like 1oz per 1qt or something real minuscule so I haven't tried it in that sense, but it does make a great fuel cleaner so maybe worth a shot.