The house and the oil cooler..
An oil cooler is designed to transfer heat. A house wall is designed to not transfer heat.
Both transfer heat from high to low.
The inside of the house is at one temperature. The outside of the house is at another temperature. In between you have materials that resist that heat transfer.
In the case of the house, the materials that resist the transfer of heat are the sheet rock, any air space, the insulation, the exterior siding, AND the air at the outside surface (boundary layer). The rate of heat transfer from the house to the cold outside is greatly slowed by the design of the wall assembly.
The oil cooler also transfers heat from the hot oil to the surrounding air. However it too has materials that provide resistance to the heat transfer process.
In this case the materials are designed to MINIMIZE the the resistance to heat transfer. However, resistance still occurs. There is the resistance of the copper tube and then there is the resistance of the aluminum fins. In addition the surface, or boundary layer of air, also has a resistance to heat transfer. The net result applies to all heat exchangers. As far as I know, there has never been a 100% efficient heat exchanger (and I've been in the heat transfer industry for 40 years).
The higher the delta T between the inside and outside air, the greater the rate of heat transfer.
So, the surface temperature of the oil cooler will never be the same as the temperature of the liquid passing thru it.
The are three other aspects to the process for a heat exchanger as well.
One includes the size of the tubes and fins of the heat exchanger. The larger the tubes, and the bigger the fins, the greater the efficiency of the heat exchanger.
Secondly, the velocity of the oil through the heat exchanger has a significant affect on the heat transfer rate. The slower the flow, the greater the heat transfer (assuming laminar flow does not occur at the lower flow rate).
Lastly, a heat exchanger has air flowing over it, either by the forced air in flight, or by simple convection while ground running. In this case the ground running is convective. As the rising column of air passed over the fins it further cools the surface area of the heat exchanger.
So no, the IR reading of your heat exchanger will not be the same as the temperature of the oil. How much of a difference depends on the design of the heat exchanger, temperature of the outside air and the amount of air passing thru the fins.