Super strange never before have I seen this happen....did a down and back run on my lake....lift the engine hatch to check stuff over as I normally do and there is a 2x3 inch hole in the side of the gear lube bottle right under the cap....wierd thing is there was still some lube in the bottom....the bottle did seem to be a little worn down in the sense of the plastic seemed flimsy and thin (looks original from 1991) but just have never seen this issue....first thought is maybe it got hot and the vent on the cap couldn't keep up....didn't hit anything prop is intact and drive was not leaking lube and I made it back to the dock and didn't feel anything unusual like vibration or hard shift.....anyone have any ideas.....was gunna pull boat and drop the gear lube to check for metal but didn't know if anyone had anything else on this??

Top reservoir cap is vented. Must be plugged .oil expands into reservoir, air needs to escape and come back in when drive isn't spinning or cooling down

Replace gear lube bottle and I would add a drive shower to keep temps in check.

the cap is a one valve persay.

Ill find a post where I comment on this.

Just buy the correct reservoir - they come with a new cap and the sealing O ring in the cap. That O ring is important as well.- there is 3 types of reservoirs as in the way the bottom fitting is position that corresponds in its mount and install the oil line. . I have them all in stock if one is in need

I'm guessing this one of the old hard plastic round reservoirs???? If so, it was probably just old and brittle.
Replace with a new square reservoir.

Correct and they cracked and fell apart on there own as well.

Hey thanks guys....I replaced the old school style bottle with the updated square one and everything seems fine....I do have a drive shower already and I think it was just old and brittle.....was just very alarming at first when it happened....worked on thousands of boats in my time as I am a marine mechanic and own my own shop in town and had never seen such a thing....all is well with it I've ran it multiple times hard and also gear lube was clean in the lower when I checked it....summer is stretching into October gotta love it!!!

Purchased a used 2008 Advantage with a Bravo one XR drive with an Imco upper gear case housing. On both occasions the oil in the Gear Lube Monitor has expanded to the point of spraying the entire engine compartment with gear lube by coming out the top of the vented screw on cap. The drive is also extremely loud with a whining sound to the point of being heard over the open exhaust. I am at a loss as to what could be causing this situation. I even replaced the gear lube and did not find any metal filings on the drain plug.

Your drive isn't going to live long. Have it rebuilt BEFORE it blows.