The only negativity of using a synthetic oil, and it's greatly debated, is that under some circumstances (and especially when synthetics were new on the market), it can super clean the internals and either cause leaks or oil burning that crud/varnish internally was masking. Synthetic oil itself will not harm any engine. The arguable problem(s) comes in with premium oil blends and additives on a vehicle that was poorly maintained or very old.
From what I understand, a big difference between synthetic and regular is how they get their multi-weight. Regular oil gets viscosity modifiers to increase the thickness of the oil. When regular oil gets hot, old or broken down from shearing (timing chains crushing the oil molecules), the oil will tend to thin out. This is really bad in hot climates on vehicles that aren't maintained. Synthetic oil goes in the other direction, it gets thicker with age/wear/shearing. No problem with that except in cold climates and building pressure on a cold start.
In all the information I've read and in the tidbits I posted above, the most important thing to know is that there's only problems switching oils when there's already problems that you don't know about or problems that are masked. It will never hurt to mix oil brands, synthetic or non-synthetic or even mix weights that are close. They will all work. The "best" is to simply use a premium synthetic oil from a reputable brand (Valvoline, Mobil 1, Pennzoil, etc) of the correct weight and change it once a year or 5,000 miles. Not to mention, a good filter. Preferably a WIX premium filter such as the NAPA Gold series or Carquest Blue.
If you really want to take care of your engine and it's longevity is your primary concern, consider researching a bypass filter. They can be a bit expensive but well worth it. Personally, if I ever have a vehicle that's not high mileage, I'll put one on. Fact is, when you get your vehicle with 150,000 miles on it, a lot of wear has already been done and if you maintain the oil, that's good enough. If you buy a new car from the stealership or one with 40,000 miles and you trust the owner with maintenance history, go for it! If your engine is young or totally rebuilt and you're building a performance car/truck that will see hot temps, I'd personally do a bypass filter with some Rotella 5w-30 oil. That stuff is heavy duty oil meant for diesels. It has a fantastic additive pack and will last in hot climates and heavy loads. With a bypass filter and that oil, I don't see why you couldn't do a 15k mile oil change.
Personally, I've used Valvoline, Mobil 1 and Pennzoil all in high mileage premium full synthetic oils. I notice no difference between any of them. My lifters tick the same on all of them and winter starts are all the same. I get whatever is affordable, usually leaning to Valvoline. Sometimes I'll try an oil when there's a mail in rebate which usually means I can do an oil change for $20. I tend to lean away from Mobil 1 because the High Mileage blend doesn't show "Resource Conserving" on the back which leads me to believe that it's not as thin and thus might not protect as well in the winter. I've got no evidence of that though.