Engine Heating up

Just as a matter of interest where are you based and what is the ambient temperature?

I too suspect the fan, if it is okay when moving then it is the job of the fan to keep air flow across the radiator when stopped. However, you mentioned temperature creeping up whilst moving, so unless it is very hot where you live, I am starting to suspect a leak in the coolant system. you also mentioned putting the heater on stabilised it, so does sound like the radiator is not doing a good job.

Are there any signs of coolant loss, or is the expansion tank filling up?

Alternatively, is the space front and rear of the radiator clear? There isn't anything fixed to the front which is stopping air flow?

Here you go, the explanation of "how it works".
In simple terms the hot air from the radiator expands the bimetallic strip, this opens the valve and the fluid locks the fan to the drive plates (as it spins). Once cold the bimetallic strip closes the valve and the fluid stops locking the drive plates (as it spins), and the fan stops turning.
This is that you see from a cold start. The fluid is still in the drive plates from the day before but the bimetallic strip has closed as the valve is now cold. You start the engine and it takes a few rotations for the fluid to return to the reservoir, unlocking the plates and slowing the fan. The AC fan is just showing this as airflow.

 

I live in Queensland Australia so can get pretty warm. It first started around Rockhampton in the middle of the day (so around 40*C) on a road trip. But it was also happening in winter down south near the snow.

The coolant was leaking before the engine replaced. It might still be leaking but haven’t investigated. Somehow recently some mud got into the system too which had to be cleaned out (yes doesn’t help the situation). The expansion tank doesn’t seem to be overflowing though.

There is the AC fan fixed to the front of the radiator. But that’s it.

It does give somewhat of a fight but I wouldn’t have thought I’d be able to stop it. I might open it up and check it out.

Just not sure which silicone oil is the best one and is available in Australia that I can pick up. Any recommendations?

I used 6000 grade, got it from my local RC car shop, was less than a tenner for 500ml.