Candace,
That is exactly what I'm doing. I've cleaned off all old grease and used Tri-Flow grease on our plastic geared machines. Especially if the plastic gear is driven by a metal gear. It seems to work very well. I won't be alive in 30 to 40 years so I'll let whoever is using them then worry about it. That is, if there is electricity to power them and anybody that stills sews then.
Joe