OK, 4.8l is what I needed to know.
Personally I would hold off on the AC lines at this point until you can get a firm understanding of what is going on there.
The 4.8l has a few problems.
Water pump can and will weep. My 2008, which I purchased in Feb had 66k on it, it is my road warrior and I am already at 80k now. I had the pump go at about 76k miles. The seal leaked and the bearings became very noise. But the pump bearings became very noise, I was not sure initially if the problem was the water pump, alternator or idler bearing. Turned out it was the pump and once it was removed you could see it was weeping out the bearing hole. Never enough to cause my low coolant light to come on, however, I checked the coolant and topped it off slightly a few times.
In addition to the water pump which I replaced, I went ahead and proactively installed the
Bimmer Fix | Coolant Pipe Repair System coolant pipe stent in the engine while I had the water pump off. I also changed over to a HamburgTech 90C thermostat as the 105C stat is really too hot for any engine. I have been running the 90C stat without any major problem, the only issue is if the thermostat starts to go soft, it will quickly trigger a P0128, which is fine as at least you are notified quickly that the thermostat is going soft.
In reality you do not need the AC for the Winter except for dehumidification. If the AC lines are leaking enough oil, the Freon will leak out probably faster than the oil.
I just hope the shop you are using is not misdirecting you on this.
And yes, coolant will not evaporate quickly, so the stain may have been coolant and not AC oil.
Sounds like you are more concerned about the garage floor, you may want to seal it and put some cardboard or a oil drip pan down to track leaks and figure out if it is oil or coolant.