i have 2 240s i'm looking at here, a 78 and an 84, both of which have different issues with their overdrive.

the 78 works great in average temps, but in the heat it will slip or disengage from overdrive randomly. i'm wondering if a heavier fluid would help.

the 84 sometimes works great, and sometimes has a significant delay shifting in or out of overdrive. the idiot light responds normally to the switch, leaving me to think the problem is in the tranny or possibly the solenoid.

 

both issues sound like the solenoid...but i would definately test first and try a new OD relay first(cheapest fix) no a heavier fluid will not help..either solenoid is "overheating" and working sparatically or you have a relay problem...od relay could be bad"weak" and the extra heat is killing it and it's turning on and off on and off...etc. get under car with car in 4th gear have someone activate the od button and see if you hear or feel the solenoid CLICK....