<VV> flushing

[cfm]

it has been recommended to me that upon taking receipt of a corvair with little knowledge of previous engine care, that I attempt to flush the engine of its old oil and filter, and add in new oil and new filter with an additive to help remedy some worn parts. Then run the engine awhile and flush again. The attempt is to resolve a possible oil leak via exhaust manifold, we are suspecting a sticky ring or two.
At any rate, I'm staring at bottles of Rislone, Berryman B-12, Seafoam, and Marvel's Mystery oil, of course, I have my suspicions on purchasing anything with the word, mystery in it.
Can some of you share your experiences, with these products? good or bad.