[5.0 Fiend]

Colapsed lifter maybe? On my last engine, I changed the oil to valvoline 20/50, and when I started the car, it had this awful knocking (so bad, my buddy half a mile away heard it), I tried playing with the dist to see if it was fuel knock, didnt help, tried pulling plug wires, no change, took it for a test drive and thought it would be safe to drive 15 miles to work the next day. On my way to work, I got tired of the noise and decided to hop on it to drown out the noise, and at about 4500 rpm it stopped, then I was like finally! The next thing I know, the car goes pop, and I look in the rear view to see a giant cloud of whiteish black smoke, got out and looked under the car, and sure enough there was oil and coolant coming out, (luckily I was about a mile from my house). As it turned out, I put a rod through the oil pan,(guess the new oil put too much stress on the motor). Oddly enough, the car started, but only wanted to run on a couple of cylinders. I guess your options are baby it, rebuild, or replace, it sucks to have your stang out of commision for any amount of time.