Reading the owners manual, it says to lightly oil 7 points on the slide/frame. Now with my Colt 1911s I lightly grease the slide rails. Why would grease in the M&P not be good? Oil seems to me to be not enough. Thanks

Grease cause more friction, and attracts/collects more dirt. I always use oil never had any issues.

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M&P 9, Shield9.

I've always used oil, even on 1911 without issue. The reason may be that there is less metal but I'm just guessing on that

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Based on experience shooting matches in hot weather, I use Brownells action grease where it slides, oil where it rotates.
Clean regularly and lube with something.

Added: If you put enough grease in a gun to accumulate or migrate anywhere, you are using WAY too much.
A frame or slide friction point should be cleaned thoroughly, and only enough grease film used that will just show a fingernail mark. More is too much.
I suspect one reason most manufacturers simply say "lube with light oil" is when too much is used, it mostly runs out...on the shooter..... on your clothes....in your holster.....but is less likely to gum up the gun.
I took apart one non-functional revolver that was packed FULL of grease, like packing wheel bearings in a boat trailer. If you can see grease in the action, that's too much.

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Science plus Art

I have used oil and never had a problem. I agree with OKFC05 and been taught. If it slides grease it if it pivots oil it. I pretty much stay with the latter. I use a good synthetic grease lightly applied.

Call me gullible, but I would assume that S&W knows more than I do about maintaining their firearms, and that they are interested in telling their customers how to get the most satisfactory service out of them. In my experience, cleaning and lubing a gun according to a manufacturer's instructions has always worked out very well.

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Pisgah

When in doubt, read the manual. The expert is the guy who reads the manual before he starts using the wrenches.

There are so few contact points and friction on the M&P, you could lube it with dish soap and it would not make a difference. These are not tightly fitted 1911's. They are cheap rattle traps that are designed to keep working with little to no maintenance.

Any light oil will do. I put a drop of oil on the barrel hood and spread it with my finger and a drop down each rail and maybe a smear under the sear.

Grease tends to migrate to places where it is not needed. I can't tell you how many hours I have spent scraping dried grease out of nooks and crannies before I can blast a slide or frame before applying Cerakote.

I put a light coat of eezox and nothing else on all surfaces of the slide, barrel, and frame rails. I probably have about 1500 sounds through it without a malfunction. I know that's not a huge round found but I'm very impressed with the M&P45k and eezox.

I also grease the slide contact areas. After every cleaning, I apply the grease (a little heavy), and rack the slide 20-25 times, then disassemble and clean off the excess. NEVER had any dirt build-up! But to be honest, I never give dirt enough time to build-up!

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Mark