1992 300D consuming a quart of oil every 60 miles

[PSDCamperVan]

So I drove to Denver, CO from Orlando, FL 2 weeks ago & made the trip fueling with B99 biodiesel exclusively as it seemed to give better performance & mileage on a previous road trip of similar length. Car performed flawlessly & never got above 80C despite averaging 80mph w/ A/C on high entire time. I tried to drive mostly at night & averaged 1K miles/day. Prior to this recent trip, I had been filling up w/ ULS Diesel #2 & oil consumption was no more than a quart every 2.5K-5K. The vehicle is a little smokey w/ RPMs above 3K but runs evenly on all 5 & seems to have plenty of power. Minimal blow-by noted under valve cover w/ oil cap off. EGR is caked up w/ soot & oil but car leaves no drip puddles when parked. A leak-down test is in order to be sure with this level of consumption. Will it be bad piston rings or valve seals or other? How far is this motor from catastrophic failure? I've been carrying a bicycle on the trunklid like a dinghy just in case ever since I discovered this issue.