904 question. Please help.

ATF+4 is fine, I don't like type F. I prefer Amsoil as it tolerates heat much better than anything else. Having said that I have Hyundai SP III in my 904 right now, couldn't afford the Amsoil at the time but it will be changed soon.

You do need to check and see if the correct dipstick and tube is being used. You may be overfilled and the extra heat is causing the fluid to expand and be forced out the vent. I've never experienced this with mine and I run a transbrake, a 4200 T/A converter and NO cooler. But mine is strictly a race car.

Do you have a deep pan and extended pickup on it? You could run that a quart low and see what happens.