I have used Lanocote.. For a flush I run it to warm, change the fluid (suck out of the top then drain off the bottom) I then re-fill and repeat, but on the last fill it gets run then over filled. In the spring I suck it all out and replace with a fresh quart. Not that I need to it is just easier than trying to suck out exactly one quart. The nice thing about these gears is they take exactly one quart. They are very reliable gears but do keep an eye on the seals and rust at the back end. The weakness in any marine gear is the output shaft / seal interface..