For the coolant leak between the engine and transmission, the most likely cause is one of the two heater hoses located behind the intake plenum right up against the firewall. The coolant leaks down from here, drips onto the back of the engine, and then drips down the transmission towards the front of the bell housing. Repair of this type of leak involves removing the intake plenum and replacing the coolant hoses with new hoses (recommend Nissan molded hoses although universal ½” I.D. heater hose will work). These hoses are notorious and the repair is kind of a PITA. While you’re in there, you might as well also replace all other heater hoses and gaskets as preventative maintenance.
As far as the oil pressure sensor light, you could have one of a few issues:
- Oil pressure is too low due to engine wear. The internal arts of your engine wear over time and create clearances that are too large for your oil pump to effectively produce oil pressure. This requires a full rebuild and IMO is most likely not worth your time or money.
- Oil pressure is too low due to oil pump failure. From my understanding, the oil pumps on these Nissans are pretty robust and I have never heard of one actually going bad… but it is possible.
- Oil level is too low, engine needs more oil. If you notice the light only comes on while braking or cornering, this is a likely cause.
- Oil pressure sending unit is failing. This sends the signal to turn the oil lamp light on when oil pressure is low. However, these go bad and can give false alarms.
With your symptoms, I suspect that your oil pressure sending unit is probably bad or that you’re low on oil. I also had similar problems where the oil lamp light would come on intermittently. I had a coolant leak as well that made its way down to the oil filter area and probably fried my sending unit. This repair is pretty easy and cheap as Satito said ($15 dollar part). All you do is drain the engine oil, unplug and unscrew the sensor, install a new one, and then refill the oil.
My first recommendation would be to not drive anywhere if possible until this is figured out. Driving with issues 1, 2, and 3 will destroy your engine beyond reasonable repair if left unchecked.
I would then start by performing an oil change (and filling to manufacturers specifications) and replacing the oil pressure sending unit concurrently (as replacing the sending unit requires draining the engine oil). This is a cheap, easy fix that (in my opinion) will most likely fix your problem.
Performing an oil pressure test would be good as well if you have the tools/gauge to do it to verify that your oil pressure is actually ok and that the oil lamp light is just a false indication. If your oil pressure is, in fact, too low, then you’ll be looking at a larger repair along the lines of issues 1 and 2 as listed above.