I have called merc racing about running merc synthetic oil in the 525,there said they recommend running there standard 25/40 merc oil because that is the oil they use and do all of there testing with,as far as using other oils they have not tested them and cannot recommend them,i use the merc25/40 in all the 525 boats and try to change every 20hrs

I have always used the Merc 25-40 oil in all of my blue engines except one boat that had twin 500EFI's. Previous owner used the "old" Mobil 1 15-50, so I used that also. Supposedly, they took all of the good stuff out of that oil, now. In my 525 EFI, I am using the Merc 25-40 semi-synthetic (not their full synthetic 25-40) ... My manual says to use the semi-synthetic. Because Merc Racing always recommended against synthetic oil, and being way too anal, I called Merc Racing techline and asked them (was either Bob or Pete ... don't remember anymore). They said that they had completed their endurance testing with their semi-synthetic and were now recommending their semi-synthetic over their regular 25-40. I used to read all the oil threads by Hydrocruiser ... he always seemed to know what was going on with engine oils and he likes/liked the Mobil V-twin oil the best, and he thought the Merc stuff was a good choice too. I also always use the Merc Racing oil filters, for whatever that's worth ... I usually change the oil twice a season and never let it go beyond 25 hrs between changes. This year I only put ~ 20 hrs on the boat, so I basically changed it every 10 hours! At that change interval, probably wouldn't matter what oil I put in it.

One of the Blue motors you havent mentioned are the 540 Bulldogs. I love mine, lots of power all the way around.

thanks for taking the time to respond guys, its very helpful. Sounds like I'll go for the semi-syntheitic quiksilver 25-40 at 20 or 25 hour intervals. . And merc racing filters as well.

Another question.. sorry guys..
Can you pull the motor's run-time breakdown from any 525 by plugging into the ecm/computer?
If I am going to pay book or close to it for a boat I want to know the engine hasnt been beat to s*$%.
I.e. 0-1000rpm _ hours
1000-2000 _ hours
2000-3000 _hours
etc.

Yes, this is how Diacom breaks it down, not sure if the merc scan tool is exactly the same.

Total engine run time: 136.80 Hours
0 - 749 RPM: 67.17 Hours
750 - 1499 RPM: 5.33 Hours
1500 - 2999 RPM: 26.52 Hours
3000 - 3499 RPM: 25.52 Hours
3500 - 3999 RPM: 7.30 Hours
4000 - 4499 RPM: 2.00 Hours
4500 - 4999 RPM: 1.22 Hours
5000 - 5499 RPM: 1.05 Hours
Over 5499 RPM: 0.70 Hours

very informative thread.

I was wondering why it would be better to use the Merc racing filter. Although it would have increased flow doesnt it have lower filtration and if youre not actually racing wouldnt a different filter be preferable?

How do you get a 525 over 5,400 rpm? That's where the rev limiter is supposed to kick in. The service manual puts top operating speed at 5,250 rpm I believe.

Not sure why it reads that way?

Probably wasn't a 525 :)