The oil temperature is not really related to the coolant temperature and usually runs hotter than the coolant. If this wasn't the case why do performance cars run separate oil coolers? A friend had an original mini minor and fitted a gauge that had a switch that went from the coolant to oil temperature at the flick of a switch, so the same gauge was used for both. Coolant ran at 170F when cruising and the oil ran at 250F when cruising. I know the transmission was in the sump, but it was also ribbed alloy to help with cooling.