Rotary Mower Gear Box ?

I was wondering what creates pressure inside one of these that did not,( from the factory ) come with a vented filler plug. I have usually changed the oil in it annually, flip 'er upside down, sharpen the blades, inspect the bottom of the tapered shaft at the seal once the stump pan is off, just periodic maintenance etc. I did not change the oil this year, looked clean enough for more run time, only takes less than a quart of Lucas 90wt or similar, calls for NLGI 000 grease in the Rhino book, its an older SE-6, but that can't be found in small quantities, seals don't leak and or have not in 11 years of seasonal use. Maybe I should see about finding some 000 ?

I initially checked the fluid level and when I removed the smaller level check plug it was pressurized and spat out a nice bit of oil. Like it was overfull. Would condensate build up do this ? I ran some more, opened the filler plug, then, the level plug, more or less leveled up too, oil still wanted to run out, so I think I did this twice, did not want too much to run out, and as I recall, it was filled to the correct level when it was changed last. Is it as simple as replacing the factory filler plug with a vented one, condensate or what, I can't imagine pressurization being good for seals, as per the book states too, but this old beast has performed well, even with some unintended abuse.

HEAT. Old snapper mowers use 000 grease. Find a lawnmower shop that buys from Stens corp. They sell it by the quart.

On a level suface, the oil level should not be above the check plug on the side. It sounds like it was overserviced and possibly still is. I never saw a gearbox on a Rotary Cutter with a drain plug on the bottom. On mine, there is no requirement to ever change the oil. Just check it by removing the small 1/8" side plug and if it needs oil, remove the top 1/2" fill plug and add 90w gear oil until it starts to run out the check plug. I've been running my 6' cutter since 1998 and have never had to add any oil to it. It's still running with the oil from the factory. I've cut 100s and 100s of acres with it. When you tip the mower upside down you're inverting the gearbox, something I've never done. I do the sharpening with it lifted up by the tractor with safety jacks under it. Use a rotary grinder. Replaced the blades once. I find I get a much better cut after the leading edges of the blades have become quite jagged from hitting rocks. Just sharpen the bluntness once in a while.
Anyway, if you have a drain plug, why don't you drain it and then refill it to the proper level and then go out and cut. Don't tip it upside down with oil in the gearbox.

The lube in it was a milky-coffee color, I welded a hook on the one side, its a lot easier to take the stump pan off, and change the blades that way, and while doing that, instead of using a vacuum pump or something, it just drains out, now I did this for several years, no issue, at all, just this year, I think its got to be water, makes no sense, it can't gain oil that is for sure LOL ! I think I'll do it over again, and see if I can find 000 lube etc. I don't know about not servicing a gearbox or any differential, I don't agree that they do not need periodic service, thats why rear ends get noisy, no one ever looks after them same with these kinds of mowers, I'd be real wary of spending any real money on a used one, unless I knew the history or it had not been abused.

I do the same with the sharpening, just block up the sides with 6x6's, and not so they can roll over, you can reach in for each blade with the 4" grinder etc.

Like you mentioned first, fill plug and level plug, it should just barely trickle out the level plug, when sitting level, kind of odd. I've run rotary cutters since I was first on a tractor, here on this farm and several other places that we owned, rented or what have you, besides the years I've had this one, thought this was some anomaly, but the tractor still being down, figured I'd see if there might be another reason, hopefully I drained enough off, and the seals are still ok, I was checking it as I was cutting, to avoid problems.

Just use John Deere combine corn head grease, it is a non fluid oil that stays in the rotary cutter gear box when the seals leak or seep.

I'm glad I read this topic, because I went out tonight to use my old 5' AC 160 mower and first thing I did was pull the level plug. It had some pressure inside and ran out a bit of sludge (looked like melted chocolate ice cream with little bubbles in it). Fortunately I had some 140 wt gear oil on hand so I opened the fill plug and put fresh in until the oil coming out was looking good.

So I figure I need to replace some seals in this thing but I have no idea where to begin. But the gear box itself is in good shape as far as I know.