[Healeys] Trans oil foaming

[Roger Grace]

I recently had the OD on my BJ8 apart to replace some parts – bearing/accumulator/gaskets/O rings etc.
For a first sort of sacrificial oil fill I decided to use a non detergent Castrol 30 wt oil. Date codes less than 18 months old. Plan was to switch to the expensive Redline MT90 after testing and after a few hundred miles. This 30 WT oil is often recommended by OD experts. I have built an electric variable speed rest rid to be able to test the transmission outside the car.
Overdrive worked fine and transmission sounded OK and ran it for about 2 hours to get the temp up (52C on OD case) and hunt for leaks.
So far so good.

Then after a few days of standing, and 2 minutes of running, I drained the oil (to fix a leak) and was amazed to see that the oil had foamed badly and there were big classic fish eye bubbles on the surface of the oil. Ambient was about 14C. I did note that the oil coming out of the transmission was somewhat foamier than that from the overdrive drain plug.
See enclosed pix.
After standing overnight the oil looked normal again.

What disturbs me is that unless I had drained the oil I would not have been aware of the foaming and put it into service .... 
Have now replaced with the MT90 that shows no sign of foaming on the dip stick.
Wonder if others have any ideas about this weird phenomenon and is it  a concern ?
rg

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