Oil for Tohatsu 20 HP outboard

[reynoldsmachine.hr@gmail.]

Owners manual says that you can not run synthetic oil in this engine. Calls for 10w40 semi synthetic. Manual says synthetic oil will cause gasoline blow by on rings.
Is this accurate or some marketing hype. For those not familiar this is the same power head in the 20 HP mercury. Tohatsu builds Mercury's smaller engines.

 

[JavierH19]

Pure nonsense. Pretty much any will do being honest. 15w-40 like delo or valvoline from walmart would be my choice.