Longbrow - You are correct that water temperature is a good indicator of what the oil temperature should be. Generally my oil temperature is 5 to 10 degrees above the water temperature. But if you are competing in any form of motorsport, even if it's once every 6 months, spending $50 on an oil temperature gauge is a worthwhile investment. Furthermore, if you were to find that your oil temperatures are spiking it would be worth spending a few hundred dollars on an oil cooler set up. Trust me, pulling out what was a good motor to replace the big end bearings or worse is not a lot of fun.
Matty