We've got the whole line of BG products at work (Nissan Service). I've got a can of MOA in with my O'Reilly (cheap) full synthetic 10W-30 in the old Volvo. I can't tell a difference, I thought I had a little lifter noise, that's now gone, but there's so many noises it's hard to tell. The BG guys are pretty convening with their torque wrench and bearing test, MOA vs. non MOA oil. Some of the other service writers really push the stuff, I've never sold a can of it that the customer didn't request, or I saw they'd used before in their history. I did some reading on the interwebs about it, and will say that, "not mixing with synthetic oils" is completely false. I mixed some in a glass and left it sit for about 20 hours, it stayed emulsified, no separation what-so-ever.
Seafoam, about the same results as most, some were big improvements, others made no difference. I do a whole can through a vacuum line into my intake about once a year. I put 2/3 a can in the gas tank and the other 1/3 in the crank case a day or two <100 miles before an oil change.
Marvel Mystery Oil, cured a tank of 7 month old separated crappy E-10 gas. A whole can of Seafoam didn't do a thing for the hard start, sputter, no start, sputter, finally come to life, terrible idle. I drove around for a couple days (had a full tank of bad gas). Coworker that used to run a engine machine shop told be to put a can of MMO in the tank. I reluctantly obliged, and within 10 miles it was darn near miraculous.
AMSoil Engine/Trans Flush. I never used it in the trans, but it sure got a lot of crap out of a relatively unknown junkyard engine. Oil was <200 miles old, first fill after swapping the engine into it's new home. I decided, to do the flush, holy cow was that some black nasty stuff that came out the crank case. I doubled the duration of the idle time with the Engine Flush in it (30 min I think) after reading some anecdotal reports of it online.