Steam/White Smoke from the Oil Dipstick?

Water in oil pan, Head Gasket BAD, check for fuel in the oil too. water may cause it to wash down from the piston area.

PCV system issue, You may also have a head gasket issue as well. if you have access to a 4 gas analyzer, I would probe the smoke/steam coming out of the stick. If it contains CO and Hydrocarbons, then you have fuel/exhaust entering in to the crank case. I assume you used new tubes and valves on the PVC system when you put the motor in. the only reason I could see that you would notice the issue when the AC is on is the added load to the motor. It's not much of a load, but it's enough to show a problem. This would lead me to believe that you have the same problem when driving down the road. If you have an auto on this car, Set the park brake, block the front wheels and place the car in drive. See if you have the same problem. If so then you have to pull the motor apart and find out which rings are lined up and causing the blow by. I don’t know if you put the motor together yourself or if you had someone else do it, but if the gap on the compression rings are lined up, they will cause a blow by situation.

Man, blow by is a definite. I am puzzled as to why it only comes out with the ac compressor on. You are applying a load to the engine, that's a fact. But blow by should be happening continuously. Leaky piston rings are to blame. It could be the piston chamber was not honed perfectly. This will cause rebuilt engines to have blow by. Also, you have to stagger the piston rings openings in order to prevent this from happening. Get a compression test or do one yourself. They should maintain the psi continuously.

If you have good oil pressure than I'd check the Crankcase filter.If it has one it'd be on the opposite side of the pcv valve, or possibly in or around the air cleaner housing..It may be soaked with oil or water..Replace it. Viola!!

blown head gasket