Hi there. I use Mobil 1 15W/50 full synthetic. I disagree that synthetic oil leads to oil leaks, and believe the benefits of reduced friction, longer engine life, and more duration between oil changes are well worth the added cost of a synthetic. The higher viscosity will provide more protection to the engine at running temp. These old FE engines have plenty of clearance so no need to run low viscosity oil.
More on synthetics: a few years back, Mustang and 5.0 Magazine reported on before and after dyno results for a 5.0 motor converted from regular to synthetic oil. Dyno results confirmed an average 5 HP increase at the rear wheels for synthetic oil, proof positive of less friction in the engine. What could be better than that?