I would advise you to NOT put transmission fluid in the oil. Its characteristics are vastly different than engine oil. On the other hand using conventional engine oil designed for high mileage a car that has seal swelling additives has helped some reduce, or even stop oil "seepage". A real leak, usually needs the gasket replaced
Common oil leaks sources are:
1. The O-ring seals on the oil cooler lines to the adapter, to the radiator, oil pan manifold, oil pan, and the dipstick tube.
2. The dipstick tube has an O-ring on it where it enters the engine case. Check tightness on all of the bolts that you can reach, do not over-tighten, that will just strip the bolt hole
The half-case leak is usually a nuisance, more than a real oil loss problem. The silicone seal is not expanding enough to seal the halves any more. Using oil for high mileage cars has had good effect, for some; use the same viscosity as normal. Same type of seal is at the oil pan.
In any case clean the entire area thoroughly before committing to a repair job.
A simple way is to spray a cool, not cold, engine with Wesley’s Bleche-Wite Tire Cleaner. This works great for degreasing, and is non-flammable. It does not appear to hurt paint, but I would be careful to keep it off of the exterior paint. If it does get on the exterior paint, rinse it with water, do not wipe off. I do this cleaning a couple of times a year, and have not encountered any problems.
1. Jack the car up and support it in a safe manner (or put it on ramps).
2. Spray everything around the side and undersides of the engine.
3. Spray the frame rails, mounts, linkage, transaxle, etc. Anything that has oil or grease accumulated on it.
4. Allow to set a few minutes, do not allow the spray to dry, or it will need to be re-sprayed for best effect.
5. Hose off using a garden hose and spray nozzle. Flush the driveway off, after the car is rinsed, to keep the residue from making a mess.
Allow to dry. Repeat if necessary after the area has dried. If you re-spray on a wet surface, the cleaner won’t work well at all.
Drive the car for a few miles then re-examine carefully. Try to determine the real source of the leak, before doing or ordering repairs.