Strange behavior
 

I noticed that when I drive the car in the winter (I know, I know, boxster in winter), that it shifts harder and is harder to get into gear when it's cold. I haven't noticed this in other manual cars I've driven, and it returns to smooth shifting once the car has been driving for about 5 minutes. Is that normal?

Also, I noticed that when I took it on a four hour drive this weekend, it consumed about 1 mark of oil on the indicator, and the aos has been replaced.

These cars have a transaxle; transmission & final drive in one unit. Consequently the entire unit is lubricated with gear oil instead of a typical and thinner manual transmission fluid. This thicker oil takes longer to warm up, and this is what you are noticing.
I could see it using a bit of oil on an extended run in cold weather.

I drive my 986 in the winter, I actually think it is a very good winter car. I have noticed a very, very minor affect to shifting before the transmission has a chance to heat up. The best advice is to not rev over 3000RPM before the car has had a chance to heat up, preferably all the way, but at least to the first tick on the coolant temp gauge. I also try to let the car sit for a minute or two after startup, especially if it is parked outside during the winter.

The oil should not be dropping at all, let alone a full bar. I would recommend testing it a few times on the same level surface. Mine often fluctuates a few bars between what it shows in my garage, and what it shows at the gas station.

I generally let it warm up for a little bit before driving it, but it doesn't really warm the transmission up all that much. As for oil, I'll monitor it more closely. I don't notice any leaks under the car, and there's no smell of oil burning or blue smoke.

Do you know what oil is in it? May be wrong oil. Mine may be a bit stiff when frozen solid but drives and shifts fine. Maybe its low?

I'm not sure if what BoxsterSteve noted about it being the transaxle oil negates this thought, but a number of people have reported poor shifting after using non-OEM transmission oil. So that may be another possibility.

Mine did this the one time I had to drive it super cold weather. Not sure what fluid is in it now, or when it was last changed for that matter, so I bought some oem fluid to swap in when I get the chance. Couldn't believe the price of it. $57/litre up here in canada!

I use the OEM fluid and drive through the winter. Our temps average between 30 and 40 degrees with plenty of days in the 20's. The shift from 1st to 2nd is a bit notchy until it warms up. There has been plenty of discussion on this forum regarding the best tranny fluid to use. My experience is nothing beats the OEM fluid.

Also check the dipstick, do not rely on the dash indicator completely.
did you wait a few minutes for the oil to drain back to the sump before checking after driving?

I wonder if the lucas oil additive would help somehow in the tranny