Jim, hello and welcome. Well, anything is possible. The pressure in all three systems can produce gradients sufficient to move one fluid from one system to the other. At idle, it is true that coolant system pressure is higher than ATF or engine oil, but, as you accelerate, your oil pressure can approach or exceed coolant pressure. I had a radiator full of oil/water when the 300E head gasket packed it in. Flush-flush-flush...Same with ATF, as speed increases pressure goes up, etc. Best bet for diagnosis and protecting your tranny and engine. Drain everything, flush cooling system, refill, change oil, ATF. Have the compression checked, and taake a peek at the plugs, for funny deposits, signs of abnormal combustion. If all is well, then you could just have some crud that has accumulated to the point where it is obvious. At any rate, find out what this is. If you have a problem of some kind, it will only get worse, just ask Larry and myself, two trauma survivors of M103 head gasketosis...
