Questions and observations:
Is it really a KMH4A or a KMH40A? I can't find a KMH4A listed anyplace on Yanmar sites. I finally found a manual for the KMH40A, but Yanmar doesn't make it easy.
I am not the owner of one of these boxes but have had several Yanmar installations and the current one (KM2P-1) has only the single scribe line on the dipstick regardless of the manual saying otherwise. I assume they printed the manual before some manufacturing cost analyst decided that not turning in the second mark would save 6 cents per unit. The logic is this: Oil on the dipstick is a must, but it should not be above the turned mark.
It is noted in both of these manuals that the dipstick should be resting on the casting, not screwed in or the reading will appear "high". Even worse, a low fill will appear to be correct. It should always be tested after the unit has had time to drain the oil off of the gears and into the gearbox sump. I check ours cold and forget about it if OK and our manuals support that. But the manual I found for the KMH4A indicated to check it cold and warm (after 10 minutes of being idle). I tend to be a bit paranoid about expensive stuff, but that seems like overkill to me for routine purposes.
Especially remember that the manual contains this warning:
NOTICE: Never overfill. The oil level must be between the upper and lower level marks.
It is extremely rare that draining the oil out of a marine gear will get rid of all the oil. If you remove it from the boat and stand it on end you might get 97%. Therefore I would never assume that draining has reduced the amount to zero and just add the specified capacity. By the way, according to the manual I found the capacity is 1.7 litres, not 2 quarts, if the manual is correct. That alone would make a 2 quart fill 11% over full.
Overfilling engines or gear boxes can surely lead to foaming and reduced lubrication. It is not quite as critical in these Yanmar marine gears since they are "splash" systems, but foaming in an engine can introduce air into the pressurized system and really make a mess bearings and such. However, even in these marine gears, it will put more load on seals and seepage into the bilge is more likely. I would never over fill either an engine or a gearbox.
I would also not consider 800 hours to be proof of success. These boxes are designed to run thousands of hours, and they might make 800 with half the oil, let alone 11% more.
