No dipstick on 1989 era 3.0L?

[Scott Danforth]

Rotella T4 15w-40 diesel

 

[Pmt133]

Yeah, try the diesel oil. I've always had issues with rotella and delvac shearing way out of grade in boats but I seem to be an exception to that.

 

[Chris1956]

If the QS 25W-40 is NA, you can use SAE 30 detergent motor oil, for moderate climates. You also might look at the OEM recommendations, as there are a lot of FC-W motor oils (used in 4 cycle outboard motors) in my local Wallyworld that might be recommended.

I never thought "Trying an oil", vs using the recommended stuff, was prudent