Water Temperature Gauge vs Oil Temperature Gauge

Unless the gauge you are considering using specifically states that it can be used for both water and oil (or other liquid material that indicated that oil may be an acceptable media), I would say no. Look at the gauge manual. Depending on the gauge, it may be acceptable for both, but it depends on the individual gauge.

Define gauge. The dialy thing in your dash, or the probe that senses the temperature.

Is it one piece?

Yes, but you would need to compensate mentally if you're using a sensor intended for water, in oil and visa versa. Plumbing fit?

the whole set sir

If it fits in the engine and can take the presure i dont see why not.

but why would you want to know what the oil temp is?

the water temp will tell you more about whats going wrong

also if its calibrated fo water the readings will be all wrong ie when water gauge shows in the red section, you oil has already been fried

by the way i forgot to mention that as the water is presurised normaly to 15 psi

the boiling point of water has been raised so the readings will be wrong for oil

Pressurizing the cooling system raises the coolant's boiling point -- it doesn't affect temperature measurement. If the gauge has a marking for "overheat", then THAT would be different. 

I have no idea if the sending units are interchangeable. You could check calibration with a pot of boiling water. (Just Google for an altitude compensation chart.) 

taken from one of my threads on radiators

http://www.arrowheadradiator.com/keeping_cool.htm

50/50 mixture of water and ethylene glycol antifreeze in the cooling system will boil at 225° if the cap is open. But pressurizing the system with a spring-loaded cap raises the boiling temperature of the mix about 3° for every 1 psi of pressure. With a 15 psi cap, the boiling temperature will be up around 265° F.

so be care full