The automobile contains a number of fluids that should all stay where they're supposed to be. Oil spots, coolant leaks, mystery drips, and other tell tale signs of fluids leaving their rightful place in the automobile are a sign to take heed of maintenance. Denying the existence of the ever-growing number of oil spots on the driveway is not going to make them go away. The source of the leak needs to be fixed.
Finger in the Dike
While oil spots on the driveway or garage floor can be ugly, an engine crankcase or transmission without oil can get really ugly. To figure out where the oil is leaking from place a large sheet of cardboard under the engine overnight. Oil leaks can be deceiving, originating at a point other than the place where the oil meanders down and finally drips onto the ground. Follow your nose to determine if the leaking oil in question is engine or gear oil. Gear oil smells like rotten eggs thanks to the sulfur compounds blended in to aid in the gnashing of gears. Tip: once gear oil gets on clothing, it will smell like gear oil forever.
That's Mr. Gasket to You Pal
Sometimes luck is good, and an oil leak is as simple as cinching down a few valve cover bolts, or replacing a useless oil drain plug gasket. Other times luck runs thin, and the dreaded phrase rear main or axle seal surfaces. The seals that keep oil from escaping past a spinning axle or engine crankshaft are of the same circular variety. The flexible rubber seal is housed in a metal carrier, which can be pressed into a transmission, differential, or engine cover semi-permanently. Replacing a valve cover gasket or oil drain plug grommet is relatively painless. Getting to and pulling a circular axle or oil seal can hurt a lot more, and may involve fun things like pulling the engine or three quarters of the suspension out of the car.
Timing is Everything
A good time to inspect and replace axle seals is when replacing axles or half-shafts. Getting to the engine seals can be a bit more complex. Take a look at the rear main seal when the clutch and flywheel are out of the car. If the seal is leaking even a little, a good time to have at it would be then. These circular seals also live behind inner wheel bearings and other places where things spin with one end in oil or grease, and the other not. To demonstrate the removal and replacement of a circular seal we hauled out a spare junkyard rear differential from a Mitsubishi Starion and put it on the table. Replacing the seal is usually the easy part. Getting to the seals can at best be difficult, and at worst give rise to a story that can now be told in the comments.