Ok, I ran a couple extra ground lines off the cluster, no change. So I busted out the multimeter and starting checking voltages at various points on the cluster and at the battery. My suspicion is confirmed, the car has a weak charging issue, noticeable at idle. I'm leaning towards the alternator being the issue.
With the car running I checked voltage points on the cluster at various gauges, when it's at 13.5 volts with hot oil and at idle, the oil pressure gauge sits at about the 1/4 mark and it's fine. But, when turning on accessories or the engine fan cycles, the voltage drops. When the engine fan kicks on it drops to 13.3 volts, add the headlights on it drops further, add the A/C blower even further. At the battery with all the accessories on, I'm getting 13.1 volts. Of course, during this the oil pressure gauge drops to zero and I get both lights.
The oil pressure gauge is calibrated to work with 13.5 volts, any less or more it throws off the reading. This is the conclusion I've reached, any other knowledge or input I'm open to.
In the meantime I'm on the search to acquire an alternator, any recommendations is welcomed. It looks pretty straight forward to remove/replace, so I'll be doing that myself.
Wow, I just bought a Spyder with the exact same problem! I'm gonna order some nice straps off Amazon! Thanks again for your time and effort!UPDATE:
I finally resolved this issue!! I never found a ground strap from the engine to the chassis. My DMM was showing a non existent ground to the engine. I grabbed a 10 gauge wire and ran it from a bolt on the alternator to the ground point on the right side of the engine bay, near the hood prop. Started the car, let it idle and turned everything on, blower on high, seat warmer, headlights and waited. Immediately I noticed the difference, once hot oil and the engine fan on the lowest the oil pressure gauge would drop to is just under 1/4 reading, and no oil warning lights!!! now at idle, with a load the alternator is keeping the voltage between 13.5 - 13.8 at the battery.
I'm going to purchase a couple 24" braided ground straps and replace the temporary wire I ran. This vehicle had this issue for a long time, likely why it went to auction a couples times before I bought it from the dealer in PA. On the face looks like an oil pressure issue, but it's not. The only thing I can think of is it manifested after the clutch change or something. I seriously can't find that ground line, I'm getting back under it one more time this weekend and look for it.
Thanks for all the input, glad I didn't replace the alternator, that wouldn't have fixed the issue, cost me about $70 because I first replaced the oil sender thinking that was the issue.