Aircraft general knowledge related question
 

Regarding piston engines

α)Why with high oil temperature we have also an "oil pressure drop" ? This is because the viscosity changes or other reason ? The opposite: low oil pressure leads to high oil T happens because the oil does not circulate fast enough-"stagnates"- so it experiences greater T .Is that correct ?

β)Does the low oil quantity always leads to low oil pressure ?? Also what can be the causes for low oil pressure ?

γ)Does the oil temperature increase have with the oil pressure decrease a fixed relation regarding which happens ALWAYS first and which happens always second ?

Many thanks in advance for your time.

Not sure if 100% correct, but have always understood...

Less oil means that the same amount of heat (energy) must be absorbed by a lower volume of fluid, therefore the temperature of the oil rises.

Quick and dirty.

A) Correct. Although these aren't the only reasons for high / low Ts & Ps.

B) Low oil quantity can mean that there's not enough oil for the pump to actually pump! Therefore you get low pressure.
Other causes of low pressure include a failed pump or even a failed guage!

C) No. There are lots of other factors to take into account when assessing Ts & Ps. For example quantity of oil, oil grade, system malfunctions (leaks or pump failure) etc.

The guages only give you a hint at what's going on. Your systems knowledge will allow you to make an educated guess from the symptoms presented.

I' ll add my share.

The usual cause of low oil pressure, high oil temperature is a low quantity of oil.

If everything is normal early into the flight and that you get such indications later into the flight, it usually (in more than 80% of cases) means that there is an oil leak somewhere in your system.

Oil has a heat-absorbing effect but before anything its purpose is to avoid excessive friction between moving and/or fixed parts.
Less oil means more friction and more friction means more heat.

It is rare for 2 gauges to give erroneous readings at the same time, so take such readings very seriously, lower RPM but maintain a good airspeed to maintain air cooling. Go to the closest suitable airport and if you start losing on engine RPM, start getting vibrations or have any other reasons to believe that you may not make it to an airfield, go for the safe off-field precaution landing while you still have options and power to control.

In the rare cases that low oil is not the cause of your high temperature, very high oil temperatures (whatever their cause) can make oil evaporate and lower the oil pressure reading.
Until the oil starts evaporating, the oil volume will increase and so will your oil pressure.