I had the same problem with the turn light, for me it was a randomly disconnecting front side turn light, the harness got toasty and was making crunching sounds at the pigtail, guessing it got too hot and aged over the years. A bad bulb in the front can cause this as well. It could be the pins are wrong, is your cluster 2nd design the one with the new quartz clock? If so I have the pin out in a spreadsheet, it's the same as 1979.
The oil sender is different, it is one wire, not 2. I think the diummy sender is 2 wire as it relies on current flowing through it when no pressure and switches off when there is pressure to light up bulb. The oil gauge uses resistance to display a sweep of measurement, which requires a different sender. It grounds to the engine so when you swap the senders be light in any sealant as the sealant could effect the ground if used too heavily. Remember avoid teflon tape when oil is involved, if pieces get into the engine I can clog things which is bad. I got a new sender it already had thread sealant so I just screwed in and didn't think too much else about it. Get a sender for 78/79, I think 80/81 used a slightly different sweep, not 100% sure in the oil gauge. I do know temperature gauge sender dramatically changed the ohm sweep between 79 and 80/81 so do not freak if your temp does not read right. The factory temp gauge is not accurate anyway. It is consistently inaccurate at lease. Explains why cars now days just show a sweep without temperature numbers.
