I have replaced 4 or 5 of the plastic UIMs over the years thanks to the fine folks on this site that have help me learn the ropes. This past week I replaced the UIM and LIM gasket on my fathers L36. Why is it that when I do this repair I always find motor oil on top of the LIM especially where the two hidden bolts are. How does the oil get up there?

Via the PCV valve.

Did you see the plastic tube that goes to the top of the UIM? That'* where the oil comes from.

So in theory the PCV valve is supposed to let out unwanted gases while keeping the oil in the crankcase and it is just not perfect in the execution? After a while would the oil get sucked into the cylinders?

Correct, correct and yea in theory, but the intake runners prevent that.

I have heard of some people trying to in-cooperate an oil catch can to stop that, which should minimize any possible blow-by.
I have never done so myself, so I am not sure how easily it can be done.
I am sure there are videos on it somewhere on youtube.

When I put my series II back together the online community I was researching said some was from the oil galleries not being sealed with thread sealer too? I know I used thread sealer on mine when I rebuilt, have yet to see any and I just pulled it back apart to check it all several months back. I also replaced the UIM with the newer Dorman part and the PVC valve was supposedly a newer revision to. Seems to be very little oil there on inspection. Thanks guys! I hope that helps too.... Bert.

Are you talking about the lower intake manifolds bolts?
If so , and you don't use thread sealant on them, then yes some of the oil could be from the oil wicking up those bolts, but the majority is from the PCV breather tube.
I have not heard of any updated PCV Valves for this motor, I have heard of the updated/reduced diameter EGR Stove Pipe, this is replaced to keep the UIM Gasket from warping the area of the coolant ports that pass coolant into the throttle body, if they warp it can cause coolant to leak.

I second what he ^ said.