I purchased a boat with a 2005 90 hp Merc a couple of weeks ago. The previous owner used conventional oil. Since I purchased the boat I have been running synthetic. I still have a gallon of conventional I keep in the boat as a spare. Is it bad to mix the two? I plan on continuing to use the synthetic but should I replace my "spare" with synthetic as well or would it be ok to mix the two if I need to do so on the water?