Only wanting to fit one tempb gauge. My gut feeling is that oil temp is more important but then most the oems use water temp and they must have their reasons. Thoughts?

If you can only have one I'd go for water temp. Water temp' responds quicker to changes in engine temp than oil temp and is directly affected by more issues. Can you not use a dual water/oil temp gauge?

Why not fit a dual gauge?

id go water temp and oil pressure if you can get one duel one

Got to be water
The water temp tends to respond much more quickly and you need to now when it's about to boil
As oil can take higher temps and is not always directly related to water temp
You'll wfective be driving blind of true engine temp

Water temp more important then oil temp. Oil pressure, is the more important reading for oil if having to choose. Dual gauges are good if wanting to keep your dash uncluttered ;-)

Fit a switch, then you can have water and oil temp.

Oil and water are similar enough scale

Water temp and oil pressure for me.

If you have some kinda transfer between the oil and water, ie: water cooler on the filter or oil cooler through the radiator, then keeping an eye on water temp and oil pressure your sorted. If the water temp rockets and the oil pressure drops you can be sure something catastrophic has happened!

If you fancy a dual one the mechanical water temp/oil pressure dual from racetech is the one I'd go for.