At least drain the old oil before adding new. Oh, and replace the oil filter. I can't stress enough to remember to put the drain plug back in securely.
Other than that, I wouldn't recommend taking the "synthetic lasts longer so I'll just wait longer to get my oil changed" approach....not saying that you would, just saying you shouldn't. I still follow the same schedule as a natural oil would use, I just run Mobil 1 instead. Good stuff, costs a load.
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