O Boy, another oil thread. The Mercury 25w40 is getting real hard to find around here but I can get the 20w50 Castrol GTX oil. Will this be OK? I've been told not to use synthetic in this motor to avoid valve train problems.

sure, vr-1 gtx kendall harley davidson oil... many opinions. you can check the marine lubrication thread. anayways i run the valvoline vr-1 20w 50. just my 2 cents. wesseldyke, who built my friends engines that run over 700hp N/A on pump gas has me run the gtx 20w-50 in his engines. so i would say you are fine

lol, I started to look at the lube thread and there are like 5,000 posts. Thanks for the reply. I'll be changing the oil often.

I have good luck with the Mobil 1 15/50, get it at Walmart in 5 Qt. jugs.

I was told the Valvoline 20w50 race oil is a great oil to run in the 525. Especially the first couple of changes. Then sinthetic is fine, but not neccessary. I use the Valvoline and change every 25-30 hours.

I have used castrol 20w50 for last 2 years in 525's. No issues yet. Of course how do you know if its good oil unless you tear engine apart to look for wear after so many hours. change every season, 30-40 hours.

Thats what I do.

Thanks guys I will go with the 20w50 Castrol dino and change it often.