If you're switching from synthetic to synthetic, I'd try and run the reservoir as low as possible before switching to another brand, but you shouldn't have a problem. Thats what I've always done anytime I switched brands of non synthetic and never had an issue. If you're going from a synthetic to a non-synthetic, I'd pull the bottom hose off the reservoir and drain it as best you can, then top it up with your new oil. I've heard stories about the two oils not playing well together in the same tank and causing engine damage. Never seen it personally, but I'd rather be safe than sorry, for the few minutes it takes to drain the tank.
