I've got a small leak under my truck that usually leaves about 2-5 drops on the driveway each time I drive it. It looks like oil but is thin. I can't see it from above the engine but it seems to be all around the top gasket on the transmission when looking from underneath. The engine oil doesn't seem to be low but the transmission fluid is low. I've only owned the truck for a couple of months and haven't changed any of the fluids left though it is due an oil change. I don't know which transmission fluid the previous owner used so I'm also worried about mixing types of transmission fluid.
Any advice on how to troubleshoot this? How about a safe choice for topping off the tranny?