I bought a 96 2500 4x4 5.9 manual with 60k miles several months ago. It had been used for fith wheel towing. First few times out after hwy runs when I got off a ramp and came to a stop the oil pressure would drop to nothing and the engine would stall. Had to keep engine reved to keep running. Changed oil, same thing. Changed to 50-20w and it has been fine since but oil gets real dark after one hwy run after change. Oil pressure gague reads very low at hwy speed in 5th gear, comes up when downshift to 4th. Uses quite a bit of oil when towing. Put a mech gague in cab and it reads about 55 psi all the time even when factory gague is fluctuating. Also usually will not start on first crank but fires immediately on second crank. ABS/Brake lights stay on all the time also. Any thoughts appreciated.