Has the major scheduled maintenance been completed? Do you know how often the oil was changed? or what oil that was with? I would be weary with changing to synthetic with the mileage. You could potentially have leaks, as the synthetic will clean the sludge regular dino oil usually leaves to help seal gaskets.
I'm not sure about 99s and spark plugs, but if those needed to be replaced sooner, replaced? Timing belt? Scheduled lubrication? Scheduled changes of fluids, i.e. transmission, differentials, etc.?
Tire rotations done every 5k miles or so? I'd look at tire wear. Were the tires rotated on a four or five tire rotation? It would be best to keep it that way, as suddenly starting a five tire rotation could be bad, as the tread depth usually would not be the same. This assumes you don't have newer tires. But even if you do, I'd check to see if the spare matches, if not, don't start a five tire rotation.
As for other maintenance, I'd look through the maintenance shedule and see what gets done at what time, and check to see if it was completed.
If you do switch to synthetic, I'd run a search on the forum. There are many threads about oil here. I'd recommend, as I believe most members here would also, Mobil 1 and Amsoil, yet I believe Amsoil synthetic is still not API certified. From what I hear, its a great oil, and you don't really have to worry about warranty unless you purchased an extended warranty for it. I doubt you'd have a problem, yet if you do, manufacturers can blame lubrication issues on the oil not being API certified. I personally use Mobil 1, on account its easier to get imo, its API certified which I feel more comfortable with as I'm still under warranty. But run a search, you'll find lots of info that will help you make your decision.
There's probably more I'm forgetting right now, but this should help some..
