Originally Posted by boxhead

You should not be rellying on the oil to control the ride height.
Ride height can be adjusted with no oil in the shock, but due to the relative light weight of the vehicle, the oil will have a degree of effect.
Also when adjusting teh ride height, push the car completly down a few times and let it settle to its height, dont lift the car off the bench then place it down and ajust from that.

It sounds to me like you may not be "settleing" the springs before adjusting ride height, so the oil is having an effect on initial settings. Then once the oil is heating up, it will thin out and is allowing your car to sit lower.

In 1:1 scale best "handleing" is with lightest springs possable (that allow required ride height), and heavy oil on rougher track.
More twists and turns then more sway bar pressure required.

I am only guessing this transfers to 1:8 scale in the same way.