If you've got a slow oil leak on a gearbox, possibly even an engine crankcase or hydraulic reservoir, consider this temporary fix to get you by until you have time to fix it "right."
Drain the oil from the component, then hook a shop vacuum cleaner up to a fill hole for the component. Temporarily seal any breathers or vents. Use contact cleaner, brake cleaner or solvent to clean all the oil from the leaky area. Turn on the shop vac, then spray more solvent on the leaky area. The vacuum in the component will suck solvent through the leak to remove any oily residue. Allow the solvent to air dry, then with the vacuum still running, smear a layer of RTV silicone sealant over the leaky area. Sometimes you can actually see the vacuum pull silicone sealant into the tiny holes that are causing the leak.
Disconnect the shop vac. Allow the sealant to cure for at least a half hour, then refill the component with oil. Compared to removing and disassembling a gearbox, oil tank or other component, it's a "quick fix."