I have used it for years to free up seized engines,and I have added a qt. to the crankcase after I was sure the engine wasn't sludged up badly (by pulling a valve cover off) after an oil change,it stopped a few engines from smoking shortly after adding it,and quieted noisy lifters...
I've used it on dozens of old small engines to un-seize them,and that is all my boss would let us use at the junkyard to get antique engines un-stuck...but usually he'd make us pull the oil pan off and see how much muck was in it first,and clean it all out and the oil pump screen,before trying to run one...
ANY additive similar to Marvel can destroy an old engine full of sludge ,especially antiques run on non-detergent oil from the days of old,that had a tendency to turn into sludge and carbon...many additives like Rislone,etc,get bad mouthed due to that,when the person using it should have checked to see how sludged up the engine was before using it...
I'm still on the fence as far as using ATF,its ok as a penetrant on stuck bolts,etc,but using it IN an engine could have harmfull effects because some types have friction additives and would act as a grinding compund on finely polished parts IMO...I have accidentally put a qt. in one engine I had and figured it was too late to do anything about it,and I haven't seen any ill effects ...yet!...I would put a new oil filter on the engine before adding marvel (I fill the new filter up with it),then after its been running a few hundred miles or hours,I changed the oil again...usually it looked like black paint!...
