oil replacement

[naviro]

I have a sundancer 340 2006 with mercruiser 8.1 horizon and I can’t find the full synthetic oil 20w-40, but I have the following alternatives:
Mercury/quicksilver 25W-40 synthetic blend
VALVOLINE FULL SYNTHETIC PREMIUN BLUE SAE 5W-40 DIESEL
What do you can recommend me?.
Thanks in advance

 

[Nehalennia]

Mercury/quicksilver 25W-40 synthetic blend

 

[paulswagelock]

Either of those would work just fine for your boat. If you still have a warranty, go for the Mercruiser branded oil to avoid any odd claims later should an unrelated failure occur and the warrantor starts looking for an escape.

If you have no warranty, the full synthetic 5w-40 diesel oil would serve you well also. A slightly better choice would be the Valvoline Blue 15w-40 if that is also available. It is a little more sheer resistant, but not synthetic.

As always, check your oil often, and watch your oil pressures. If the oil sheers a little, the pressure will drop. Rule of thumb says 10 psi for every 1000 rpm is in the safe zone for a warmed up engine.