Castrol Syntec
 

Any of you guys use this? I did my 10 hr change yesterday and used this oil in 10w30. The only confusing part for me was the lower unit change, It calls for hypoid 90 but I could not find that anywhere so I used Lubrimatic 80w90 hope thats ok! It says its for outboards and is high viscosity......

I don't think it hurts to use synthetic, other than some say it lengthens the time to fully break in the motor. FWIW, I use Castrol GTX in my 4s. Coincidentally, when I bought my oil filter, the mechanic also recommended that same oil. He must have felt strongly about it because I didn't ask for his advice.

I normally use Lubrimatic LU oil, and sometimes use regular name brand gear oil. Your owners manual will indicate the minimum API requirement for your crankcase and LU oil.

Bill, NAPA has the 90 I think.