I'd stick with dino till the rear main is done-- if it leaks now it will leak after the change, and possibly quite a bit more. My 940T with 200+K mi
seems to use a bit of oil, particularly when it is in boost a lot. Mobil 1 is about $8 Canadian a quart here these days, so that adds up. The rear main drips a bit on mine too, which has kept me from synthetic.

Some friends have noted that transmission shops are often cheaper for rear main repairs than general automotive repair facilities. They do it every day and don't seem to see it as quite such a big deal. These cars are a cakewalk for them-- they are used to doing front wheel drive cars with no room to work. Call around a bit-- $800 is a stiff price for a rear main seal.

I swapped to synthetic on my '92 245 4 years ago now-- not a drip of leakage, and it looks really clean when I peek in the valve cover now. A massive difference compared to my 945T which has been on a steady diet of dino. I didn't do anything before changing, but I did swap filters after about 1000 mi. This car uses less than half a quart of oil between changes, which are twice a year. Not too bad for a car with 145K mi.

Good luck!