dip stick abso no oil.few miles down the road the rockers stop knocking and gauge runs normal.dip stick shows full.cold start it's empty again.rechecked few miles later,its full again.this is a repeating thing.what could cause this? bad oil filter,oil pump or rear seal going bad? never experienced anything like this before.oil/filter changed yest.08 f150 4.6L 8cyl.

 

So you checked oil, had none showing on stick, started it up anyway? then heard bad noises, but drove anyway? for a few miles? Not good.

 

Lmao...

 

Sounds like the oil is being trapped up in the valvetrain area. Probably restricted drains in the heads. You can fix that when you rebuild the motor after running it without oil circulating.

 

I would park it on dry newspaper to see if its leaking. Hole in oil pan? Common to have filter leak at seal ,especially if old seal still stuck on flange . What does the bottom of truck look like is it coated in oil ?
Is your exhaust full of smoke .
Sounds like oil pump is working but its filling up drive train . You have to be careful getting plastic from oil bottle seals in there , it could block oil returns . Is one side noisy and not the other .
You could try a motor flush but really this sounds serious blockage . How much sludge could there be in there.
Does this engine have some kind of oil cooler on it ? Dont drive it .
Sounds like you are going to have to pull oil pan and check pickup screen as well as pulling valve covers . I really want to see what you find .