Have a 1939 Farmall H that ran great and had great oil pressure until I tried changing the transmission oil. Now I've killed it and am desperate for help as I dearly love this tractor. Here's the story: My brother was supposed to drain out old trans oil while I went to town to get new oil. IH-Case dealer recommended 6 gal. of Hy-Tran Ultraction Hydrolic Transmission Oil, which I bought. Got home, brother was gone, but tools were everywhere and plug where trans oil goes in was out and the hole was open. I assumed he had drained oil, so I started putting in new. A little over two gallons in, oil started coming out of "level check" plug hole on side of tractor. Brother came home just then and said he had not drained oil! Stupidly, I figured, "Well, if it was two gallons low, I'll just put plug back in and call it good for now." I know mixing oil is bad and shouldn't have done this - I goofed. Now, tractor starts up and runs fine for five minutes until it is heated up. Then it starts to lose power and oil pressure drops from 3/4 where it has always been to just above 1/2. Engine seems to "miss" too now after heated up - never did this before. Also, after heated up, trans oil leaks bad from cover plate below tractor in front of where one of the trans drain plugs is located -- seems like oil coming out of the open holes in that plate (I'll attach a pic of the area where leaking now). Odd thing is if I let tractor sit for an hour, it starts up fine and oil pressure is back up and it runs fine for five minutes again. Help - have I truly killed this tractor? What should I do to get it back to running great again? I'll be forever grateful for advice and recommendations.