I'm thinking just the fact you gave it fresh new high quality oil with quality cleaning additives is starting to help loosen a sticky exhaust valve or lubricate a worn stem seal. High mileage oil mostly just has seal conditioners as an additive, otherwise they have the same lubrication properties and SAE ratings. Any additional wear additives are mostly just advertising IMO or else they'd be in all their oil. High mileage oil is generally not recommended for engines in good shape seal-wise. The seal conditioners cause the crank, cam and any intermediate shaft oil seals to slightly soften and pucker. For a worn seal that's fine, it helps keep oil from weeping past the seal and defers the need to replace it. For new seals they cause premature seal wear, shortening their lifespan. For red blocks, the front seals are easy to replace, so a little premature wear is no big deal. The rear main seal is a different story, difficult to replace and often goes past its best before date and starts to weep, as seen under the bell housing. With my high mileage red blocks still on their original rear seals I do occasionally use high mileage oil when it's on sale. The tradeoff is I use the best front seals I can get and I've got no problem if they need to be replaced a year earlier. If the front seals in the white blocks are more difficult to replace then I would avoid high mileage oil.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now