Oil Weight

[mr geets]

I bought some Castrol GTX 10-30 for my 90 refi 4 stroke 2011 Merc as it is off warranty now and I thought I'd save a buck. Caution set in and I went to the dealer and bought 25-40 Mercury oil. I did my annual oil change yesterday and inside the engine cover the temperature range showed +10 to +100 or something like that for the 25-40 oil. I had concerns that 25 weight viscosity was pretty stiff for water that is 40 degrees early in the season.
Coincidentally I had a relative pick up supplies from another dealer. This morning I checked it and it has Mercury 10-30 oil?????

 

[Chris1956]

I would expect your motor to call for FC-W motor oil, which I have been led to believe is more robust than your garden variety automotive multi-viscosity motor oil. I have used the MerCruiser 25W-40 oil in real cold weather, without incident.

I would use the auto oil in the auto, and stick with OB oil in the OB. You can get FC-W oil from multiple manufacturers, so you are not tied to Merc.