Punto,
You are about to discover something we all find out sooner or later. When someone, even your instructor, tells you something, it is possibly not correct. The engine oil temp is not the reason for your takeoff power limit, get your money back from that instructor! If the engine oil temp served as the limit for the takeoff power (thus limiting the helos performance, gross weight and payload) then it would be fixed by adding a bit more cooler capacity, believe me!
The take off power limit is most often set by the life reduction that comes with the stress on the engine (usually the crank or the exhaust valve temperatures) and the stress on the gears in the main transmission (tooth bending). Could be either, or both at the same time.
This thread is traveling into an interesting direction! For those who advocate limits observance, welcome to my position. For those who think, as long as they observe the limits they are doing no damage to their helicopter, welcome to the real world. You ARE damaging your helicopter by observing the limits. That is the message that I have been discussing, but that Gomer has bent into some other black and white case.
Limits are not magic lines, where below is pure and white and full of happiness, and above is awful and black and full of death and destruction.
Observe the limits, you must, but as you do, be assured, even as you do, you are damaging your machine. It is forming little tears in its metal, it is struggling to stay together. It is extracting beans from its jar.
What is the difference between above and below the limits? The difference is that the RATE of damage is more understood, and the ability to get closer to assured life is more sure.
Does that mean if you reduce TO power at 5.0 minutes per application that the engine will not fail? No, but it gives you a better chance.
All pilots are requested to observe the limits, but let all pilots be aware, there are many more limits not expressed, and just as likely to get you in trouble.