After running dino oil most of the life of an engine you should not convert to synthetic as this will clean out all of the deposits and/or sludge blocking your seals, or making the seal, and your old seals that have dried out due to being blocked by sludge and/or deposits now don't seal anymore. That being said, you'll have to replace whatever seal is leaking now to fix it as its a one shot deal generally, once that built up "seal" is gone its gone.
I run Royal Purple 20w 50 in the van after the first 1000 miles on dino oil, Royal Purple Max ATF in the recently rebuilt transmission, and Valvoline Sythentic gear oil in the 9" rear diff, on the van.
Merc gets dino oil 20W50 in the engine as it is a slow leaker "good running" short block I bought for $300 that i put in there, no sense spending dough on the synthetic for that one. She also has the same Royal Purple MAX Atf in the tranny and valvoline synthetic in her rear diff.
SRT-4 only ever gets Mobile 1 5W-30 in the engine.
'76 F150 I have gets whatever is avail, cheap, or heavy, 20W50 most of the time, she drinks her daily fluids but never misses a beat!
