To answer your one question. You should change your oil every 6 months. When oil sits after it has been run. It will form acids and other compounds that are not good for your engine as they say.
Alot of people have switched to full synthetic on high mileage vehicles with no problems.
As for seals drying out. Start the motor once a month and leave run for 30 minutes. Better yet. Go for a ride.
This will keep the caliper pistons from sticking.