My car did exactly the same when it had a water leak. The oil temp would start to rise and only after I'd lost a good bit of coolant would the engine temp start to go up. The cause of the leak turned out to be the water pump. I also changed the thermostat as well, if you do change the stat get it from Nissan (£17 + vat) as I bought one from my local car spares shop and was given the wrong one, it fitted but looked really cheap and nasty the Nissan one was much better quality. Plus I fitted a Nismo Rad cap which was about £20 off Ebay. To monitor your coolant to see if you have a water leak, try the following.
First of all fill and bleed your coolant system by making sure your car is on a level surface then remove the bleed screw, its on the thermostat housing, there are three bolts holding the thermostat housing on and the bleed screw is right next (to the left) of the top bolt.
Once the bleed screw is removed, remove the rad cap and fill your rad with coolant until coolant starts to come out of the bleed hole. Try to be quick and refit the bleed screw while the coolant is still pouring out of the hole.
Once the bleed screw is tightened, carry on filling the rad until it is full then replace the rad cap. Also fill the expansion bottle up to its Max. mark.
Now drive your car as normal and keep a watch on that Oil temp gauge, if it starts to go over its normal temp you will know that you have lost water again thus telling you that you definitely have a leak.
This method of determining whether you have a leak or not will only work if its a relatively small leak as with a larger leak the coolant could pour from your engine quite quickly (especially under load) and cook your engine, but as long as the leak is fairly small and when the temp starts to go up you refill the coolant straight away and exactly as I've explained you should be o.k.
Obviously make sure that you are carrying plenty of water with you!
Incidently I replaced my water pump and my temps are fine now.
