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Hi all. I am running crusader older 270hp crusaders. I’m using mercruiser 40 weight oil. Someone said I should be using multiiviscosity oil, like 15w40. Was wandering what others are using and what is recommended for small block motors.thanks Mike

Mike-
I use 15w40 in my older Crusader 454s. I know of many strongly held - and shared - opinions about what is the right oil to use. In the end I decided to use an oil analysis company and take their data and direction over others' opinions.
Maybe send a sample to Blackstone Labs https://www.blackstone-labs.com/ and get their recommendation?
Best,
Stephan

I use Rotella 15-40 in my 440's.

Not sure how old those crusaders are, but back in 1975, Mercruiser recommended Straight 30 weight for the 454 blocks. Then Mercruiser changed over to multi weight oil. I believe it was 10/30. We ran that for years with no problems.

I would think any good multiweight would work fine. Unless you are in a very hot climate, you may not need 15/40. But it will not do any damage if you use it. Less chance of the oil breaking down on a long run.

I've been running the Mercruiser 15/40 oil with good luck. It is a little more expensive but given the frequency of changes I've stuck with it. I have also heard many have had good luck with the Rotella as Rawleigh indicates.

Straight 40 weight in 440's for 39 years,
24 that I had them and I pulled them out running strong.

Brand- -any and all quality brands. The only one I disliked was Valvoline as it broke down a bit quicker which was a problem as my rear mains leaked a bit when oil broke down.

Have to say I think the important part was oil change frequency.

10w-40 Amsoil Marine in my 230 Mercuisers.