I have an old ford ranger the muffler is bad, but does it sounds like it...

...backfires too is this normal? when it needs a muffler. and the air filter has some like oil on it

The oil in your intake is probably from the rings on the pistons going bad. combustion gasses are leaking past the rings into the crankcase and blowing oil through the vent in the oil fill cap and into the air cleaner housing. If the muffler sounds rough, but the engine itself seems to be running smooth, you just need to get the muffler done. If the engine is shaking, or the backfire is like a random poof or pop, then the engine needs more attention then the muffler. The Cat. converter may be getting plugged up if you're burning oil also.

If a motor doesnt have any resistance in the exhaust, It will make that sound. The muffler or catylitic converter create a little back presure that smoothes your motor out. GET IT FIXED