The baby oil I buy is near pure paraffin oil, which is distilled from petroleum oils. There are synthetic mineral oils, which a lot tend to use, not sure on the base being from plant or petroleum. Out here you can also buy it in bulk form from Bunings as Diggers Paraffin oil. There may be vegetable based oils out there, but not the ones I buy. The reason for just getting small bottle is it lasts me many years, and I like the small aperture on the cap. I also use it to thin out bearing blue. I have never noticed my indicators or mic spindles to get stiff over time.It is not the paraffin that is often confused as being Kerosene . It is a very light highly refined oil that is used in industry alot, as well as in the medical field. You can ingest small amounts pure paraffin oil and not be poisoned. It's in my nasal spary for hayfever for example.
All that really matters is the repeatability of the instrument or tool. You can easily test the different oils etc, and will see that with a thicker oil, it will read differently to a very thin oil, or when it is clean and dry.
The testing I did, clean and dry was the same as using the baby oil and still wet with Brake cleaner. The car oil (20W50) made it read late by 0.015mm on my testing. Baby oil is Zero, Clean and dry is zero. Wet with brake clean is zero, ATF oil is zero. My lathe oil which I was certain would be late , as it has 20w50 oil, some ATF oil, Lucas Oil stabilzer(really sticky stuff) and wyns friction reducer, was also zero and did 1 read of early by 0.005 the resolution of my mill DRO is the X axis. 10 weight , very light spindle oil, also Zero. Lastly tested 60 weight approx castor oil, it was late by 0.035mm . It was also the most difficult to clean off the surfaces after testing. It was the stickiest of all oils I had for testing.
My conclusion is , you can use a lot of different oils and some mixes of oils and still get the same results. If it gets sticky though or gummy, then it will read incorrectly. My edgefinder is the type with a 10mm diameter ceramic disc running on a steel shaft. I also have a Starrett one that has a 6mm shaft, but did not test the steel to steel one. It has bay oil between the surfaces.
Edited By Neil Lickfold on 20/06/2022 00:36:23