Mobil 1 15W/50 or Fuchs 10W/50, i use the latter, these are both popular choices!

You need to measure the power at the pump WITH the engine under load, i.e. a voltmeter wired to the pump inside the car, whilst giving it a bootful. You should ideally see alternator voltage, but anything less than 12V is the danger point.

The wiring in the fuse box certainly sounds dodgy and I would advocate investigating it!

Martin