A few months ago I traded for an M&P 40 fullsize. I put 30 rds through it and locked it in the safe and forgot about it. Today I got the urge to shoot it. The only ammo I could find locally was tula 180gr steel case. The first two mags ran fine, then I noticed that the slide started slowing down when returning to battery. Slow enough that not only could I feel it, but I could watch it also. Eventually I had 3 rounds out of 100 that didnt feed. The bullet noses would be pointed upwards, but lightly pulling back on the slide and letting go would send those rounds into the chamber.

Any suggestions? I'm not new the M&P, but I've haven't had any issues with them until now.

First thing I'd try would be different ammo.  Brass cased.  Give it a thorough cleaning beforehand.

I went and cleaned it. It had been cleaned before I shot this morning and lubed with gun oil that came in a cleaning kit. This time, I used wheel bearing grease. I dumped two mags as fast as I could pull the trigger and keep it on target and it worked fine.

This has me confused. Would a quick cleaning and using grease instead of oil make that much of a difference?

the M&P 40 is a very reliable pistol.

.40 tula steel case is not very good ammo

I've run 1000's rounds without cleaning mine, but since I feed them with a DILLON xl650, they are well broken-in

Tula is filthy ammo and the steel cases allow the crap to build up on the chamber walls since the cases don't expand enough to adequately  obdurate  and prevent it.

That's why cleaning your pistol got it to run as it should; I don't believe using the grease rather than oil is what got your pistol running right.

My own M&P .40 has been exceptionally reliable with brass cased ammunition.

My M&P 40 FS will fail to feed 180gr FMJ .40s&w once or twice a magazine. Sometimes I can tap the back of the slide to put it into battery and sometimes I have to eject the mag then pull the slide back to clear the malfunction. The ammo used was always brass cased and from various manufacturers (WWB, UMC, Blazer, etc.)
The first time I took it out I put 50 rounds of WWB 180gr down range with no problems but after that every trip to the range resulted in the malfunction I mentioned previously. I never contacted S&W about it because it was never a CC/HD gun, it shoots 165gr without a hiccup, and since picking up a 9mm OEM barrel I rarely shoot .40s&w much anymore.

I did have a M&P 40c that ate everything and asked for seconds so I figured the full size barrel must have been made on a Friday.

Today I ran 200 rds of various 180gr brass cased ammo through it with zero problems.

I like it, and it's more accurate than my Glocks. Any recommendations on carry ammo?

My M&P40 is extremely reliable, I can't remember any hiccups at all as a matter of fact.  Crappy trigger from the factory but shoots everything I've fed it.