This is a subject I have avoided over the years since there are so many negative claims about 3in1 oil. In the past it was used in many home sewing machines which had plastic parts with no ill effects that I am aware of.
I’m not an “expert” on lubricants so I will make no claims along those lines. I can only give my own experiences over the past 20+ years.
3 in 1 has been the standard lubricant on the Port Orford Coast R.R. for that period. I use it on my Tenmille plastic switch throws, on the metal axle ends (journals) of my plastic side frame trucks and on the metal pins in my plastic Kadee couplers. I have not had one single issue with the oil damaging the various plastics in any way during the 20+ years I have been using it. I lube the components usually each Spring and the wheels roll beautifully, the couplers operate smoothly and the switch stands operate flawlessly. Not one single failure due to the oil. Just ask those who have run trains here over the years. I have not used 3 in 1 on locomotive gears as something a bit heavier seemed more appropriate to me.
It is my understanding that you are not supposed the use regular oil over or in conjunction with synthetic oil without completely cleaning the latter off. Perhaps that could cause some interaction or something, I don’t know.
I’m not trying to argue a point as it makes no difference to me what others prefer to do but I find it remarkable that over many years with claims made against 3 in 1 on plastic that I have yet to see a single photo of damage done to a part by the oil although there have been some due to a lack of lube. I would like to see such a thing if it exists. Too, perhaps there are some plastics that are affected by 3 in 1 and if so I would like to know about it.
So there you have it, one person’s experience with 3 in 1. If you decide to try it I suggest experimenting on a limitedl scale at first and expanding to everything else once you feel satisfied. Maybe my results aren’t typical or perhaps hotter or colder temperatures will have an effect.