First post on here, so hopefully this is a good place to post such a question. I inherited a broken 1998 K1200RS from my friend. I'll give you a quick rundown of what happened, but if you want to skip to the end and read the current and specific question I have, feel free to do so.
Last summer, I was riding it and it started to overheat. I pulled over and turned the bike off. After that, it wouldn't start. My friend brought a trickle charger and we got enough juice in the battery to ride it home (about a 30 minute ride). First thing I did was replace the battery, and then it would start up (however it continues to die if I don't have my batter tender hooked up when it's off). However when it did start, it would belch white smoke out of the tailpipe (I know, the most likely candidate seems to be blown head gasket, but I'm trying to cross off easier things first). So the first things I did was change the oil and the coolant. Before doing so however, I noticed when I checked the oil through the little glass port, it APPEARED there was none in there (yes, flat surface, center stand, warm bike, etc). That said, there was plenty of oil that came out of the bike when I went to change it (at least 3 quarts). Changed both the coolant, oil and oil filter without incident.
Now we get to where I am now and my current question. I turned the bike on to heat everything up and push both fresh coolant and oil through everything. Although it seems a little less, it's still spewing white smoke when running. But the stranger thing is right after I put the oil in (3.7 quarts, which is what the manual calls for when replacing the filter as well) the glass oil level port was full. However over the course of a few minutes, that level dropped, and dropped and dropped until it looked just like it did before the oil change; that is to say, it appeared there was NO oil in it.
Nothing is leaking out of the bike, so I'm not sure where all the oil went. I guess my main question is does anyone know what this behavior is indicative of? Do I just need to put more oil in until the window is full again (that would be about .5 to 1 quart MORE than it says you should ever put in it).
Also, for those who read my full sob story, any suggestions people can give me in general on what they think might be going on I would be eternally grateful.