I don't know for sure, but my ranger (1990) has an oil pressure gauge, not a gauge that measures the amount, and i would guess that your's is the same, so when it is atr half, that means that your oil pressure is just right, but i can't say why it goes to zero, you sending unit may be bad.
as for the tranny, the synchros sound like a probability, but i would change the fluid first, it may be low or to thin by now
as for the gas thing, i would fix that asap, check for a general hole or maybe some rust, or, if worse comes to worse, take it to your tranny friend and have him patch it up for you