The standard LU oil from BRP is fine for that motor, the synthetic is better. We use the full synthetic Merc lube in race motors. Even running a V4 gearcase on a Mod-VP V6 we found almost no sign of wear.
I use Quicksilver Hi-Performance Lower Unit gear lube but there are others like Royal Purple 75w-90 gear oil, Amsoil Synthetic Gear lube or Mobil 1 75w-90.