Car is shaking, dont think it is the mounts

[JarydW123]

I recently replaced the A/C compressor fan in my january 79 240D, I also replaced the compressor belt. I also did an oil change. When I was replacing the belt I realized that the compressor fan belt didn't have a tensioner bolt (like the fuel pump), and I would need a pry bar to tighten the belt. I met up with a friend of mine about 20 minutes away, the car was driving fine. I tightened up the belt and fired it up, it was fine at idle so I started driving away. When I was idleing in drive the oil pressure dropped dramatically. I shut the A/C off and it only went up moderately. I went into the nearest walmart and added oil. It did nothing. I brought it to my mechanic and he said it didnt look like the mounts. I did notice the oil cap was leaking slightly, so I'm going to get another O=ring for it, hopefully that will solve the oil pressure problem. The car does have a fair amount of blow by which indicates it could have some compression issues, but it seems strange that immediately after I adjusted the pulley is when the problem started. I just need some insight before I get a diesel specialist involved. I bought the oil cap today and it didnt help. The oil pressure doesn't drop until the car is warmed up completely, when its cold it still shakes but the oil pressure stays up.