My hydraulic system on the dump trailer has Dextron II in it. NOW when I put a different pump on it ---it will be a used pump that was running regular hydraulic oil or trans/draulic type.
Question is, will the new used pump be ok running Dextron in it?
It would be hard to get all of the trailer system drained,--- then again maybe not bad. ---Main concern is cold weather use of the hoist. I use it quite a bit in the winter to haul manure home for the gardens and it does get cold on the road.
The trans/draulic type works o.k. in my tractors and thats what I kinda wanted to run.
If I disconnect the cylinders and push/pull them---most of the Dextron should be out. Will any remaining mix without problems?
I know some oils wont mix!--did that on an earthmover one time and had a wicked mess on my hands, so dont want a repeat of that ordeal! lol!

in your dump trailer system Sonny the oil use is intermittent, the Dextron/ATF light fluid will work fine,it probably doesnt even get worked hard enough to get warm. If it were a system that sees longer duty cycles and heavy pressure loading I'd shoot for a heavier fluid(oil) like a 46ISO to maintain lubricity and carry heat off from the pump back to the tank or exchanger..the bigger issue you have here is flushing the old fluid out but you said you were going to evacuate the system and drain what you can,the little bit thats left will probably be a very small amount in the hoses that didnt make it out,maybe a dip in the hose where a portion of what was left in the cyls didnt make it past into the drain bucket..no big deal. Dirt ingress would be the biggest concern when you open the system up. Anyway that Dextron is funny stuff, I havent seen a Dextron fluid system yet that doesnt weep or drip a little bit somewhere from a fitting or two..auto fluid purge so to speak

Probably best to leave Dextron in it since thats whats been in it since new.
Ya, --- it would only run 5 mins. per load so not much heat would be generated.