Oil: Synthetic blend vs regular oil

[SST]

I am going to change the oil in my boat. I bought 25W-40 regular oil. I cannot find the receipt for the last oil change and I'm not certain if the current oil in the engine is Quicksilver synthetic blend or regular oil.

What I need to know is, if I put the regular oil in, will it harm the engine? Not all of the old oil gets pumped out so will these two mix safely?

I recall discussions about autos that once you put synthetic oil into the engine, you can't go back.

 

[Denny]

The only thing that I have read is that the 2 do not mix, as to what harm, I do not know. I found this on the net.

Synthetic oils typically provide better protection than conventional oils, but switching back and forth between full synthetic and conventional oil will not damage the engine. Of course, this depends on the current engine condition and the quality of the conventional oil being used.

 

[Doc]

I had my transom replaced last year at a performance marine shop. So many impressive boats. I had them winterize and change oil in mine and they used Rotella. I told them I had been using Mobile One synthetic before he changed the oil. Later I talked to him about how synthetic kept my engine temp 5 or more degrees less than dino oil. Less friction. He said optimum performance the temps need to be up within a certain range and synthetic can prevent that. He races his boats and knows of what he speaks (I think). So I'll have him winterize it again and change the oil using Rotella.

 

[dgiles]

There is zero harm switching between synthetic and non-synthetic oils. I would not mix them, but again, that still would not harm anything. How do you think they get semi-synthetic? Rotella 15w-40 is a great dino oil for boats or use what the manual suggests. Most legs call for full synthetic and I would not deviate from that if that is what they are calling for. for example, Volvo Penta sterndrives take a 75w90 Synthetic gear oil that meets GL-5 rating. Any oil that meets this is fine.

 

[SST]

I had posted a reply above, I see that it didn't take. I'll try again.

I had visited my boat the other day thinking I had a 1/2 quart of engine oil left. It wasn't there as I'm pretty sure I poured it into the engine during the season. So then, I called my mechanic and asked him what he usually puts into a Merc 350 and whether it makes a difference. His reply was, "Either is fine." So, that's what I'll do.

The only "mix" that will occur will be what oil remains in the oil pan that I can't pump out.