Well for starters I would would get a oil pressure test gauge. Hook it up with the adaptor for your eng. (going to be a tight fit) then you will know once and for all if its a presure problem.

Then you will know if you need to hassel with oil supply.
If you have presure then you will know you need to go the electrical route.
If you suspect a ground even though like you said not likely to only affect oil sender. Just add a jumper cable from your body to eng. block. that will put that question to rest.
But you mentioned you had sometning smoking in the past? Under the air filter housing? I would want to know what that was asap. you may have to pull the gauge cluster and mesure the ohm. resistance in you oil presure sender wire.

So I guess my point is do the oil test and you will know what direction to go.