oil temp is important here, not water... imho the best thing is oil temp gauge, so when oil temp reach about 80 or 85 degrees celsius you can flor it safely... 85-100 degrees is good oil temp for driving. and I never turn off engne if oil temp is higher than 90 deg c. I think oil temp (measured in sump) is for about 30 degrees lower that temp in bearings... I heard that somewhere...