I have an oil leak on an 85 12a that is coming from the bottom of the Oil cooler/filter (this is the setup where the oil filter sets on top of the oil cooler and not the version where the oil cooler is located under the radiator). Assume this is the "O" rings. But the leak only occurs when the engine is warming up, e.g. the leak seems to stop after the engine is at operating temperature. Has anyone else seen this, does this make sense?

 

Most likely O rings as you were saying. Appears to me, when it warms up the seals expand and stop the leak. I would replace as it will get worse I am sure. Better to catch it now then when it is leaking all the time.

 

bad o rings, they are probably leaking when it's cold because the oil pressure is so much higher then, when the engine worms up the pressures decreases and the leak stops.