I have a 13 row on currently and have found that temperature rise of the oil after three laps (either oran or wakefield) results in equivalent rise in water temp. ie , ordinarily both run at 180degF, but when the oil temp goes to 220degF, so does the water.
When used on the road I don't have this problem.
I put it down to the fact the turbo is oil cooled and the full throttle/hard braking heat soak in the engine bay in general is a problem.
Stick a temp gauge in the sump somewhere, you can by NPT adaptors, and check what temp it runs at. Ask the oil company, of the type you use, what they recommend. I don't think 120degC is going to hurt the oil, but it may affect the water temp as described above.
As a note, I use HPR 40 (the 25/70 one), anything else seems too thin for the gearbox and it starts making horrible noises, even when cold. I've been running this since 98 and the gearbox is still together
Cheers
DOZ