Fred,

The Parker O-Ring manual  shows that ATF and MIL-5606 have similar rubber compound compatibility. The only significant difference is that the MIL fluid compound is 75 durometer and the ATF compound is 70 durometer. This is likely due to the convention of commercial and automotive compounds being specified as 70 and 90 durometer and MIL compounds being specified as 75 and 95 durometer. Old habits ....

The short answer is that for the hydraulic gear and flap system you will probably be safe going with the empirical experience. You may need to replace seals after 5 or 10 years but probably not.

My concerns are for the other use of MIL-5606, the brakes.  The first concern is compatibility with the aquarium tubing. There are lots of reasons not to use un-reinforced nylon for a brake line, and this adds to that list. The second concern would be vapor pressure. How do boiling points compare? Third is bulk modulus. This is primarily due to the fluid to absorb and maintain air in solution. Finally there is water absorption. Both fluids are hydrophilic.

ATF also comes in different flavors. Which flavor used is likely important.

I would recommend you do some research on the topic and report back.

Anticipating the next question, you should NEVER use automotive brake fluid in your Lancair brakes. You will rot out all the seals in short order. The Parker ./ Cleveland components are designed for MIL-5606.

Finally, MIL-5606 contains the additive Tricresyl Phosphate to increase lubricity at high contact pressures and low speeds. Tricresyl Phosphate is a known irritant and carcinogen. Use gloves when handling. Nobody wants "Fried Chicken of the Skies".

Regards
Brent Regan