Low engine power?

And I know this is a bit of a joke but will any sort of additive help? A tech from justanswer recommend try marvel mystery oil on a oil change

Ok and should I rotate the engine with the plugs out a couple times before I start it? Just to make sure the lifters are all pumped up a little? I've never touched lifters so I have no clue, and is it a sure bet on a high mileage cam?

Oh and if I burnt a valve wouldn't it miss/backfire? The exhaust note is irregular but to an untrained ear it isn't a very pronounced difference, I would expect a burned valve or one that isn't seating to be far more noticable. Am I correct on this?

Alright do I just let it idle or rev it up on start up? The rev up I think is just for new cams right?

One last question, will the lifters come right out of the guides or do I need the remove the guides to get them out?

And again thank you so much for your help, you clarified a lot!

Is there any way to verify its the lifters? I called a Couple Chevy dealerships and none of them heard of replacing the lifters while replacing intake gaskets

I did the LIM gasket and the upper plenum gasket and I changed tge plugs, now its got a top end on cal 3, and the car has low power
Compression good
Oil pressure good
Spark/plugs good
Injectors in working order
Vacuum is good but at idle it sits at about 19-20 inhg and it fluctuates constantly 3-4"hg

Symptoms
Low power
About 22mpg avg, was at about 25-27mpg before this happend
It has a bad hesitation on acceleration
Rpms usually sit at 750-800 at idle but now it will drop to about 550 when brake is applied in drive and will go back to. Normal when foot is off brake(intermittent)
Valve clatter very pronounced on cold start ups
Doesn't want to hold revs when it hesitates, it gets to about 2 grand on accel and will just drop. Off and make tge car hesitate
Rpms will jump 200-400 when I let my foot of the gas at cruising speeds 25-45 does it all speeds but more noticable at 25-45
I will occasionaly have power randomly but after the car gets driven for like 20-30 min it hesitates and has low power

I do have power if I make the car downshift, but it still doesn't have the power. It used to.

Iv done
New intake gaskets
Plugs/wires ac delco
Air filter
Seafoamed it (helped a. Little on power/mpg)
Oil is clean
Coil good
Maf/map good
Can't find any intake leaks
I feel like ive been running in cirlces, but I'm like 99.99% positive its a valve train issue, any ideas on how I can be 100% sure what it is before o tear into it again

Would it be safe to try a quart of marvel mystery oil on an oil change to see if I can get it pumping up? And can a single Lifter be replaced? Can I it be replaced without removing the intake manifold?

What would you try first before blowing 500 bucks digging into it. Again!

Dang when I pull the lifters do I have to remove the guide? And what's a recommended procedure for prep of new lifters and a break-in precedure? Or point me out a place to find this info

Sorry for a lot of questions, but is it damaging to drive it like this for long?

I'm curious is it possible to misalighn a pedestal mounted rocker arm? My car isn't breathing right at all. And I have a pretty pronounced valve train noise from a lot of my rockers. If I pull the covers again what should I look for? On cold starts the valve train is noisy and exhaust is a loud puffing noise, no miss just noisy and a major hesitation/lack of power.

This started after doing a lower intake gasket.

Ps there's no coolant that goes through the intake manifolds

Any testing tips would be great instead of saying go to a mechanic. I'm here cause I would Love to fix it on my own over paying a 100 dollar diagnosis (which I've done 2 times to hear them say "we don't know!") Its not worth it.

3.5 v6 flex