So, ordered 4 gallons of OEM Merc 25W-40 4cycle oil for oil change on my two 525efis. Open the box, and they shipped 2 gallons conventional as ordered, but mistakenly sent 2 gallons of 25W-40 4cycle Synthetic Blend.

My question is, while the manual only refers to conventional, would the synthetic blend mixed with conventional be a mistake? Blend in one engine and conventional in the other?

I prefer to follow the manual to a T, but wanted to bounce it off my OSO buds!

Thanks....

No better test than a side by side comparison!

reading what I can, it appears maybe the Synthetic Blend is for Outboard 4 strokes. Particularly, the Verado Racing outboard was mentioned....Hmmm

You'll be fine use it. Make and attempt to get as much of the old out but you'll be fine. Opinions are all over the board on this and many switch to sythetic. If you paid for reg and got synthetic I wouldn't call them out on it by any means.

It just exceeds your required conventional oil in a sense. Better under higher heat temps, rpm's, less varnish and holds its viscosity better. I believe most of mercs racing engines require it. just stick with it moving forward so you don't get a mix of the ongoing. Little won't hurt anything.

I used the synthetic blend for a while and then started to have issues with lifters. But can't say for sure if the oil was the problem. If you ask opinions on oil, you will get 1000+ different answers. Here's 1001.. I now use Valvoline VR-1 20W50 conventional in my 525EFI's and change oil and filters religiously at 25 hours.

Just to clarify. For several seasons, I used the conventional 25W40 Merc oil in my 525's. Then the Synthetic Blend 25W50 came out and the back of the bottle says, use for everything from 525's up to 1350's, so it sounded good to me and I used it for one full season and then most of the following season and then had two collapsed lifters. Now, I also changed valve springs in between and the springs I used were too stout, so in all likelihood, it was the springs that were the problem and not the oil. But in the course of my troubleshooting the lifter problem, I was guided by some very reputable guys to use an oil with a lower cold rating and to stay with conventional. I ended up with the VR1 20W50 and ran it through two oil changes last season. Seemed to hold up well and engines ran like kittens.

Me two. Just placed an order yesterday to stock up.

Conventional VR1 20W50: http://amzn.com/B00DJ4FIPQ

So, called the guys at Merc Racing Cust Svc. He said they are cutting all their oils over to the synthetic blend, so the conventional is being phased out. I like and prefer conventional with regular changes as HyFive578 mentioned. The every 25 hours scheduled that Merc has put out seems to be a formula that folks are really sticking to, and if they are not, they better start! I have 29 hours on my current oil, so overdue!!!!

Thanks guys!

Here's 1002. There is alot of good oils out today. Straight petroleum (crude) - Blended petroleum/syn - Straight synthetic. My belief is, what is added to the oil. One of the reasons that I am a believer in AMSOIL. Also a believer in using a Zinc additive, no matter what oil your using. With the exception of the below mentioned oil.
When Gibbs Racing first starting running the Toyota's, they had valvetrain issues. Solved with adding Zinc to the oil. Joe Gibbs racing oils was born.