It's totally fine, given that you:
1) Empty the oil before you put new in(obviously)
2)Use the same weight recommended for your vehicle
Now as for switching to synthetic from regular motor oil...sure synthetic is better for the environment, it lasts longer(resists break-down), and keeps the engine cool etc..but if you have a high mileage motor, and you have some small leaks, it may leak even worse with synthetic. Synthetic is a bit thinner. So, it flows better in the motor, but it might flow right out of the cracks and out of your engine faster as well if you have leaks.
Hope this helps.
As long as the oil meets the most recent API SN and ILSAC GF-5 standards and your car makers' specs you'll be ok.
Do NOT change the viscosity. Read this article on why it's a bad idea
http://ricksfreeautorepairadvice.com/mot...
Put in the brand you like best. William is right about the leaking, if you have some leaks, use a high mileage oil, instead of straight synthetic.
Changing brands is acceptable. You should be good as long as there an ASE approved stamp on the product you're buying.
Is changing brands of motor oil in an old, high mileage motor acceptable? What about the weight? Thank you
