In a 2 stroke... there is no pool of oil to lube the lower half. It's mixed with the fuel, and leaves a film. That makes me think about what Lou is saying. (Leaky seals) If this is the original 16 year old crank... it's probably time to replace it. I hate to be the bearer of bad news... but I've seen it a bunch of times... especially with the higher HP engines. The top end gets replaced... and the old lower half gives out in a few rides.
OK... in the center of your engine, there is a set of gears that get oil from the injection tank. The oil in the gear case should not be mixing with the internals of the engine. At face value, a leaky center seal is superficial. BUT, when that is coupled to an old crankshaft... it's a sign that the bearings could be warn to the point that they are moving. (not holding the crank true) The next problem will be that your end seals will be leaking (air) and that will cause the top end to cook it self because of a lean run.
Now... coming back to your comment about the oil being "Greasy." That's not good. If there is water in it... there can be water in the counterweight gears. This is a 2 fold problem. First, the water could be etching the center bearings... and rusting the counter gears. Both will lead to a short life.
If I'm off base here... I will keep to myself. I just don't want to see someone put the $$$ into a new top end, and have the engine blow because the clues for other damage were over-looked.
In your 800 engine, there is a service port in the wall of the crank housing... in the PTO jug. You can put a hunk of wire down it... like a dip stick... to see the condition of the oil. If there is any sign of water, the engine case needs split.
