In the summer in Washington DC area (hits 90s F ambient temp), I saw oil temp in the pan (drain plug sender) around 240F on the highway, and it would quickly jump to the 265F range with a little boosting. I was probably in the 10-12psi boost range at the time, nothing crazy.
I did a track session once on a small road course and I pegged the 300F oil temp gauge...backed off for a couple laps, it dropped off 300F, and I went back to chasing a WRX.
It's worth having an oil cooler on a stock car in my opinion. On my daily driver I only have the oil/water heat exchanger as used on the later 940 Turbos. I do not have oil temp readings, unfortunately. But...we raced 24hr lemons with a 240 F+T and another team successfully ran a 240 w/Turbo at ~10-15psi with only that oil/water heat exchanger. On our car, we had an oil cooler from a 240 Turbo (Setrab, the good stuff)...but somehow they never cooked the thing with just the heat exchanger sending oil temp heat into their coolant.
The air-cooled oil cooler was a no-brainer from my perspective....although it did cost us an engine due to poor placement when it got taken out by debris. I relocated it to the driver side headlight (near stock 240 Turbo position) and it was reliable there, out of harms way.
For what it's worth, the stock flexible oil cooler lines can be cut apart to reveal a barbed fitting and then -8 line can be clamped on. I did this on my autocross car for front-mounting the 240 Turbo oil cooler. Repairs in the field become easier when you only need to carry spare harvested fittings and some lengths of hose. Even 1/2" heater hose would work in a pinch.