I've been using a hi-visc non synthetic lube in my lower unit for a couple years now and was wondering if it would be alright to drain it and use a synthentic type lube (Mercury brand) instead. The motor is a 1999 Johnson 90 hp. I'm asking because I think I read somewhere it was not good to mix a synthetic and non-synthetic lube. Remember I am draining the old lube so will there even be enough of the old lube left to even worry about the mixing problem.<br /><br />Thank in advance