Although I own a 116 chassis, I'm fairly certain that your model also has a mechanical Oil Prssure gauge. There is a black very rigid plastic line that runs through the fire wall to the back of the oil pressure gauge at the back of the instrument cluster. Either the plumbing has developed a leak or the receiver for the gauge has.
If that is the problem, you may be able to find another cluster at a salvage yard like Potomac German Auto. You should then be able to transplant the odometer assembly (this all presumes that it wasn't contaminated either).
S, J.R. Brown
2000 G500 LWB Obsedian Black
2005 Toyota Tacoma Access Cab Off Road Sport
1993 Volvo 240 Sedan Anthracite
1980 450SEL Champange (owned it for 15 years. Great car)
1986 280GE LWB Anthracite (Sold it and kinda wish I hadn't)