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I've been running Motul 6100 semi-synthetic 10W-40 in my M21 for over 7 years. Im at 186k miles and willing to pamper my M21 a bit more but stuck at finding a more premium oil. Primarily that so called ZDDP stuff but honestly its hard to track what does (or did) have it.

Redline recommends their 15W-40 but my instinct is to continue using a 10W-40 , or go with the 15W-40 for the summer but switch to the 10W-40 for winter driving.

Amsoil recommends one of their 5W-40 diesel oils.

Any thoughts, including just continue using a semi synthetic with frequent oil changes and maybe adding a bottle of a ZDDP additive?

Hmm, not so knowledgeable about this. Let me ask our neighbor, he'd know about this.

I searched on this forum and found only a couple mentions of using a 5W-30 or 15W-40 here and there.

I decided to try the Redline 15W-40, March is already getting warm and last summer was a scorcher.

Yup, that's definitely a scorching month. My neighbor has no say about it as well. Sorry about that.

I've been running Chevron Delo 400 15w40 in mine. Not for any really special reason beyond I can get it locally at Napa. When I had the rocker cover off to do the timing belt and lash the valves, the cam looks fantastic. Unfortunately that was all prior to me miles and i have no idea what sort of care this engine got in it's prior life. I got the car with unknown miles and history beyond knowing it sat for about 10 years. I fixed the digital odometer and it told me 186k. No idea if it was actually logging miles, imagining them, or really anything else. The car does not seem to have that many miles but ???

Gadget, that is true mineral oil right? What is your oil change interval?

yep, standard dino oil. I'm not opposed to synthetic and I run that in my gas stuff. I'm due for a change in about a thousand miles, and I may go synthetic at that point if its not just way too expensive. 2 gallons of oil and that filter makes it not especially cheap.

5k miles, which is what the Ford manual says. It ends up being about every year to year and a half. I don't put a lot of miles on it and the change it as much for age as miles. Everything I own gets changed on that kind of schedule and for the same reason. I don't go anywhere and I own 4 vehicles to do it with. This year I expect the diesel will get even less miles. 2 of my cars weren't tagged last year. I have a DMV appointment in 2 weeks though so the plan is to get everything legal at that point.