I found that full synthetic during brake in is a bad idea in any engine. The engine will not ware enough to fully seat piston rings. I run dyno oil for first few miles or hours to get engine broke in and then switch over to full synthetic after break in. Synthetic is so superior against engine ware and ability to take heat it pretty much stops metal to metal contact. I can not run synthetic in 4 stroke raw water cooled outboard motors because the engine will never get to normal operating temps and start collecting moisture in the oil. That's one more thing that proves how well synthetic oil works. Amazing stuff.
Delo or T5 work just fine, you just need to change them more often. On the other hand some newer auto-gas engines require full syn. Don't know if it's true but apparently bearing tolerances are tight enough to require 0-20W oil.