I have a '91 40 hp Mercury & I'm not sure of its past. I thought it might be a good idea to flush the lower unit. Is that a good idea, & if so, what should I use to flush it?
Drain it and refill with fresh lubricant.....if the lubricant drained out looks contaminated then you could run the unit through the gears for a few minutes and drain and refill again....but these units are sealed and as long as the oil seals are good the fluid usually stays pretty good except for a bit of darkening caused by normal wear.....
