According to the Mobil 1 website, you do not need to wait for a break in period before switching to synthetic oil. They list several manufacturers that use synthetic from the factory. I swear by synthetic oil, and on below zero mornings when my X starts right up and is ready to roll, it confirms that belief. I started using synthetic oils in my wifes 95 Toyota Land Cruiser which has 145K on it. No problems what so ever, even the filler opening looks cleaner! Go for it, it will be the best move you can make for your Xterra.