I have a 04 Bayliner 175 with a stern driven Mercruiser 135 hp motor. I was out the other day and noticed that all my stern drive fluid was missing from the reservoir. I went in and bought some Hypoid 90 drive fluid and filled it back up. I took it out again to check and noticed a "oil slick" coming from the stern drive unit on the water. I checked the reservoir and it didn't appear to be losing fluid at idle so I ran it across the lake and back and checked it again, "around 5 to 10 mintues out and back". The reservoir was nearly empty. I didn't see any fluid at all in the bilge area so I'm assuming its leaking from the lower unit itself. My question is, would this be a seal leaking in the lower unit? If so how hard would it be to repair? I would take it down to the local marina but the last time I took it there they had the thing for 3 months replacing a stern drive unit that had been stolen. If I have to take it to the Marina any guess as to how much it would cost? I know its kind of a vague guess at this point, but I assume a marine mechanic with lots of experience on this forum might know on average what it costs to fix a leaking stern drive unit. Also I am afraid I ran it for a few minutes when it was almost out of fluid before I noticed it was empty and on the run back to the boat ramp. Would doing this once or twice ruin the gears? Don't want to take it some where and have someone try and sell me a new lower unit if I probably haven't damaged it by running it out of fluid for a short time. One last question. What exactly is Hypoid 90 drive fluid? Is it similar to 50W motor oil? I looked closely at it and it appears too thin to be similar to 90W gear lube for cars.