On a house from the mid-1960s, the usual suspects are the torsion spring, the galvanized lift cable, and the roller track alignment - all three age out around the same point in a door's life, and a 1966 build is well past that point if nothing's been touched. We also see openers that were swapped once in the 1990s or 2000s without a matching spring adjustment, which throws the whole balance off in a way that just feels like the door is "getting heavy."
The honest limitation: if the spring, cables, and rollers are all failing at once on a door this old, patching one component just buys you a callback for the next one within a year. At that point we'll tell you straight that a full door replacement is the better spend, even though that's a bigger ticket than a maintenance visit.