I wouldn't worry about switching every 6k.
I've run for 15K since switching to synthetic and the oil still looks good.
The purpose of running synthetic is better lubrication, but more importantly longevity.
I'll probably change mine when I hit 50,000 or 60,000 miles only because I do a lot of mudding and water crossings in the spring time.
Keep in mind any damage to the rear was already done during break in, so changing the oil more often won't help now
