A friend of mine was having starter trouble with his 88 2 wheel drive. I did the aux starter relay, like I did on mine and it solved my problem, but on his it caused the oil light to come on.

Any ideas?. We changed where we grounded the relay but still the oil light came on. Plug the factory wires back together, and the oil light goes out.

Any clues?

Why would it work on my 89, but not on his 88?

 

Obvious answer is it's wired wrong.
Can you make a schematic of what you did and post it here?

I installed a relay for the starter on my 91 and only touched 1 wire from the existing wiring / harness.

 

no, it is not wired wrong. the 4 pole relay is pretty simple and I double checked the wiring on my truck after the problem on his. you have ground, power from battery, wire from switch, wire to starter. even at that, the oil sending unit has one wire going to it, just like my 89.