My 83' Turbo also recently had a huge oil leak. It caused smoke at highway speeds (and non highways speeds) beacuse the oil would hit the exhaust system, and then be drawn under the car out the back. It turned out to be the brass connector between the oil pressure sending unit, and the block. This piece was cracked, causing oil to come out at 60psi. Also, any smoke on upshifts or high vacuum situations can be caused by leaking valve guides, and I believe this is what is currently happening on my car. Yesterday I changed the oil to Mobil 1 synthetic 15w 50, as the thicker oil seems to leak less on my car, and maintains a higher oil pressure.