Well I had a slightly different problem on my 82 240 M46. The OD would work when it was warm but took forever when the temperature was down in the 30's or 40's. I finally traced the problem with some help from this board to the OD switch. I checked the switch and found it worked(but not right away when cold). I just replaced the switch today and the OD will work as soon as the car goes into 4th gear.

My point here is to make sure what the true problem is. I have had problems with the ground wire near the solenoid break and had to make sure that connections are good on the solenoid. When was the last time you change the fluid in the transmission? Are you sure you don't have a problem with the OD relay?

Do not under any circumstances put the heavy oil in the transmission.