Bad valve seals?

Normally it's just on startup from what I have seen. But if when you add new oil it does it also kind of sounds like they could be bad.

Before you compress the spring, grab a deep well socket larger than the diameter of the keeper circumference and that sits nicely on the valve spring retainer. Then, once centered over the valve & keepers but ON the retainer, give a few sharp quick raps with a hammer on the socket. The spring pressure combined with the raps with a hammer should loosen them. But, know in advance, that if hit too hard or at an angle, the keepers can come flying out and the retainer can come off quite easily (and if on the engine, the valve could fall in the cylinder). If your doing it on the bench, no big problem. But, if on the car, you want to tap much lighter to loosen before pumping up the cylinder with air. Its one of those things that experience teaches of how hard to hit without releasing them. However, you do have to hit it pretty hard to have them release and fly across the room. But, a good blow with the hammer usually pushes down on the retainer over-coming spring pressure for a fraction of a second, the keepers move just a bit to loosen, then the spring pressure pushes back up on the retainer after the hammer blow very quickly ... but this has then allowed the keepers to be jolted a bit and thus loosen. Sometimes one will come out or get wedged outside of its seat, but then they have been loosened. As long as one is in there, then you can compress the spring and remove them. If one comes out, and you lift the socket off the retainer, it can get lost very easily on the bench or in your head if doing on the car ... so be careful when you lift the socket off the retainer and be ready to catch a keeper or put a rag around things to catch it. Did it many years in a engine building shop. And definitely then inspect the insides of the keeper to make sure they were not damaged and all edges remain in tack because this can damage the insides of the keeper groove edges if hit too hard. If any edges have been damaged, those keeps must be replaced to prevent dropping a valve!!! And, also beware that if you really hit it hard enough, you could crack the top of the guide installed in the head. Ideally, you do not want any of the keepers to literally come out of the retainer when you hit the socket with the hammer onto the retainer ... on the bench is another story since it is an easy way to remove all springs very quickly. Any springs that I want to remove on the car, I go around the engine first and hit them with a socket on the retainer firm enough to help loosen them but not enough to disengage the actual locks and send them flying! You want to hit it so that you overcome spring pressure for just a second and only a couple thousands of an inch actual drop in the retainer. Hope this helps!

Just take the heads off and have them valve jobbed and bowls cleaned up by a professional.
Worth every penny and is good for quite a bit of power, they will also take care of the spring seat pressure too.
Should only cost about $200.

With that said, oil burning because of valve seals is pretty rare, unless the heads sat for quite a long time.
Usually it's the rings, bearings or screwed up intake gaskets.

I just did mine a couple weeks ago. They're pretty easy, the hardest part was gettin the keepers out, was a pita. I just tapped the top with a hammer before compressing the spring. Then put the spring compressor and compress it as much as possible, then use a good magnet and take the keepers out. It goes pretty quick once you get going.

Oh and the rope through the spark plug hole works great, put the cylinder at tdc and shove as much rope in as you can.

Yes, the rope in the cylinders works very good and prevents a potential break in seal when air is used. I used this method many years ago. Thought it might be a bit more difficult on a 5.0 though due to having the right size rope and feeding it in there ... you must compact it as you go to insure that the whole area is filled with rope. Too large a rope and it doesn't fill the cylinder properly and then gets too hard to push in ... and too small a rope and it takes a year to fill the cylinder and pack it. However, putting rope in the cylinder and then turning the engine by hand until it binds up against the valves sounds like a good idea! Not sure if this might pose any problems getting it all out ... sounds like it would work great. Anyone done this and then turned the engine over by hand to compress it against the valves and then remove the rope successfully?

With only 20,000 on the motor I doubt it is the valve seals.My guess would be to check the intake manifold gasket.

Do a leakdown test.

Remember with the right stuff, once you crack the bottle open, you are using it or it's drying up.