I have several questions OK the first one has to.

I have several questions OK the first one has to do with oil. One day I checked my oil because my truck was running funny and like lerching around then it quit for the rest of the day but anyways I checked it and it was dry no oil. So I filled it up and have been driving it everyday and whenever I slow down my oil pressure gauge drops down below 40 but not under the white line its in between.I also was at a stop at a stop sign one day and I smelt oil burning and so I got home and rested the truck for an hour or three and I checked the dip stick and the oil hasn't moved at all on the dip stick. But it hasn't been bone dry on the dip stick ever since that one day. What could that be caused by and what parts do I need? My second concern is I tried starting my truck the other day and it clicked and I popped the hood wiggled the thermals tried starting it "click" then I hooked it up to jumper cables with the other truck "click" so then I just decided to keep turning the key repeatedly then it finally turned over but it only done that one day and that was like 3 days ago it hasn't been an on going advent but the oil problem has been but for the starting part of the problem what's going on and what parts do I need to fix it?