15w50?

15W50 is definitely too thick to be running for the winter in your climate. While not a recommended Porsche viscosity, for summer use in Arizona, that's one thing, but for winter in Virginia it's not a great idea.

You need to determine where the leak is coming from before assuming it's the RMS, as it may just be a valve cover or oil pan leaking. If the leak is coming from the joint between the engine and gearbox, it is likely to be the RMS, though it could also be the IMS seal leaking. The gearbox needs to be removed to determine which, and to allow access for the repair. If you do pull the gearbox, it would be a great time to update the IMS bearing and replace the clutch, as the added labor is negligable.

The concern with allowing the RMS to leak is that eventually, as the leak gets worse, it will coat the clutch with engine oil, causing significant slip, and rendering the clutch worthless.

Often use would be defined as at least twice a week. Anything mechanical doesn't like sitting around, and that includes cars, motorcyles, airplanes, boats, locamotives, etc.