Can I mix regular oil with semi synthetic?
 

Good Morning,

I have a 05 496 ho with 600 hours. I have always used Mercury 25-40 oil. I placed my order for oil and accidentally ordered 2-1 gallon of the semi synthetic and two quarts of regular oil. Can I mix the oil or should I stick with the regular oil? I have not had one problem with the motor(I hope it stays that way) using the regular oil.

Any suggestions would be appreciated.

Thank you,

Lou

Mix away. Motor won't know the difference.

Exactly! We’ll get bashed 😂 but it’s true

my experience
 

I broke a oil pressure gauge adapter one time and lost a bunch of oil, we scoured the docks looking for folks with some spare oil, suffice it to say we got a few different viscosities and brands but all pretty close .
That oil mix was a mess, foamy and aerated....crappy oil pressure . I remmember thinking I should never mix different brands or viscosities ever again.
That being said I think you`re fine but personally I no longer mix

Thanks for the replies. It is the same weight oil. I have no idea how I ordered semi synthetic in place of regular oil. I’ve been ordering for 19 years the same oil.

I know people argue about the quality of oil but I have heard if go with full synthetic you shouldn’t go back to regular. Don’t know if that’s true or it applies to semi synthetic

Most semi-syn's are way more conventional to syn in the ratio. So adding a bit of conventional of the same brand to a semi-syn shouldn't be a big deal. And if it is the same brand the additive packs should be compatible.

Dan's example is pretty extreme and yeah, wouldn't recommend doing that.

I had a 400 hour 496 that I ran almost exclusively on M1 15W50 but there were a couple times Wally World didn't have it so I ran VR-1 20W50 and then went back M1 when it was available. Never an issue. When motor was torn down at 430 hours the bearings looked brand new.

Unfortunately there is not much of a quality regulation that determines when semi-synthetic means. As mentioned....its likely more conventional oil than synthetic. Same brand, model, and viscosity, will likely have the same additive package.

Run it.

The dino 25w-40 is what Mercruiser recommends of course but doing semi-synthetic likely won't hurt, especially as you have two quarts of "the good stuff". In my other boats, still GM powered, I ran full synthetic with no issues...usually Mobil 1 15W-50. I have read and re-read and re-re-read the passages in my Mercruiser Service Manual regarding oil and don't entirely understand WHY they are so anal about only dino oil, at least in the 7.4MPI engine...but I guess they have their reason(s).

I believe that to be a myth ESPECIALLY switching out with same brand. Everyone has their favorite and that’s fine. Likely what they’ve had the best luck with but doesn’t necessarily mean brand b or c wouldn’t perform the same.

I’m still using Napa oil (which is suppose to be valvoline) in my boat and muscle cars. Synthetic in my newer cars and trucks that specify.

Like others have said…mixing oils likely won’t harm anything.

personally I wouldn’t do it unless I was in a pinch…aka low on oil at our destination and had to go hunting for some just to get home.

peace of mind is worth more than $25 in oil.