By "new" engine, I guess that means new pistons and/or rings ? If so, take the car to the nearest long,(at least half a mile) fairly steep hill and give it full throttle all the way up, in top gear. Start at about 30 mph and increase to 50, let the car slow back to 30 and do it again. And again. And again. Keep an eye on the temp gauge - it'll probably get warmer than usual but don't worry unless it gets near the red. Use the brakes to moderate the car speed if necessary. You may need 4 or more runs up the hill to do any good. Black, runny oil in a new engine after so little use is almost certainly caused by the cylinder bores "glazing up" (a glass-like surface forms on the cylinders which the piston rings can't seal against) due to the new rings not bedding in properly. Bursts of full throttle, moderate revs driving creates high gas pressures in the cylinders, forcing the rings out against the cylinder walls. The resulting friction "beds" the working surface of the rings against the cylinders. I'd do it pretty soon and not use the car at all until you do. Its possible that glazing has already gone too far. If so, I'm afraid the only proper remedy is to pull the engine apart and re-hone the cylinders, using a glaze breaker or a ball hone (some people suggest feeding a fast idling engine a fine abrasive such as Brasso to roughen things up again. Tried that in desperation. Disasterous! Glaze still there, piston rings ruined.).

Re. the tranny leak, the only proper cure I've found is to replace the gaskets. You can make them yourself with oil-jointing paper if you're careful.Hope it all works out.