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