Mixing Stern Drive Lube viscosities ?

bigdog · 09-30-2021, 09:25 AM

Just drained and refilled my Merc 4.3L sterndrive.
I bought 1 qt of Mercury SAE-90, but it was not enough. I can see this in the engine reservoir container, that it is about 4-6 oz, below the full line.

SAE90 is not cheap at $16 a quart. Question..... I have some 85W x 90 drive oil I used last year for a Outboard, can I add this to the current straight
SAE-90, without causing any issues ?

I'd buy another quart of SAE90 to add, but I'll be selling the boat in the Spring.

Thoughts?

09-30-2021, 09:25 AM	#1
bigdog Senior Member Join Date: Apr 2006 Location: Central MA-Gilford Posts: 1,459 Thanks: 357 Thanked 126 Times in 100 Posts	Mixing Stern Drive Lube viscosities ? Just drained and refilled my Merc 4.3L sterndrive. I bought 1 qt of Mercury SAE-90, but it was not enough. I can see this in the engine reservoir container, that it is about 4-6 oz, below the full line. SAE90 is not cheap at $16 a quart. Question..... I have some 85W x 90 drive oil I used last year for a Outboard, can I add this to the current straight SAE-90, without causing any issues ? I'd buy another quart of SAE90 to add, but I'll be selling the boat in the Spring. Thoughts?