Sounds like the oil could definitely use a change. I'm no pro, but I've always used 90 weight in my sm420 and I've had no problems. With you being in the colder climates I would be curious to know if you could find a lighter oil for better shifting.
I haven't heard of methods for cleaning out the gearcase, though I would warm the tranny up before draining. I always filled my tranny up to the fill hole. Not to say that's the right way, just what I've done.
Best of luck,
Bo :o
