Re: Oil temps way high after header install
You'd be surprised how little header and header proximity effects oil temps. Many years ago (before we made headers) we had a road coarse track guy who had a set of F-R headers that made contact with our pan. Not just a little contact, but primary contact along a +6" surface ON the pan. He had no overheating issues. NOT what I'd ever recommend, but it certainly showed how little the header effects the areas around it. If you you look at flow, volume, surface area, etc. a worse case scenario and you're exposing <2% to higher temps.
I would check your sender connections/ground. If they were disturbed during the header install that could effect the temp reading on the gauge.
What else changed when the header was changed? Oil change? Tune? Fuel type?