low/no oil pressure, no start

Hey, I had this problem a while back, and somehow the engine started up and ran fine for a while, so I didn't think much of it, but now it's back.

couple weeks ago driving down highway, temp got about half way up on gauge and oil dropped off to nothing. pulled over, saw real thin oil so I put in a few quarts, but didn't help, turned out when it got to certain temp the oil pressure dropped off. So I cahnged the oil and serpentine belt (unrelated, it just squelled) and didn't start it that night, but next day went to crank and it hit for a couple seconds and died. This has happened before, but it usually started up after cranking a lil more.

there's no white smoke, it will kick with some ether but sounds crappy, and when I unplug the sensor on the driver valve cover same thing(I believe oil temp? I remember doing this before to check if bad sensor). the HPOP is low, and I can fill it but it drops off again. The tach is registering a little bit (maybe 100-150 rpms) when cranking, but it is turning over plenty strong

I'm thinking oil lift pump, but not sure, any thoughts?