There is a lot of people that have opinions on this topic… these are my own. If you follow my recommendations, you can’t go wrong.
Sewing machine oil is a fine grade of oil that does not contain any additives that will deteriorate or deposit varnish on the oiled parts. The brown film you commonly see on the needle bar and presser foot bar on a vintage machine is this varnish and it caused by using the wrong type of oil. Always use sewing machine oil for your regular maintenance.
Oils to use… I use a brand of oil called “Tri-flow” . It is a high quality oil that contains P.T.F.E.. P.T.F.E is Teflon and it has one of the lowest coefficients of friction of any solid. It’s drawback is it is more expensive than other sewing machine oils, but I think it is worth the extra cost. Any other sewing machine oil is fine.
Oils NOT to use… Do not use 3 in 1 oil, automotive oils, any penetrating oil, light household oil, silicone spray, kerosene, or WD-40. A word on WD-40… I love the stuff. it is slippery and leaves a protective film. helps free up stuck screws and mechanisms, and helps clean the old oil, grease, and varnish on parts. WD-40 is NOT a lubricating oil for sewing machines. The “WD” in WD-40 stands for “Water Displacing” and this is what it was formulated to do. After ultra sonic cleaning, I soak all of the parts in WD-40 and it will suck out all of the water trapped in tight places and leave a protective film. It does not offer long term lubrication.
How about grease? Grease suitable for gears is not suitable for small motors. If you are re-greasing the gears, do not use automotive grease. Use a product recommended for sewing machines. You won’t find this in any retail store, but a sewing supply store or the internet is the place to go. I use Tri-Flow grease, it’s good stuff on gears but NOT in motors. The grease recommended for sewing machine motors is designed to melt at a given temperature. The motors use a “wick” system and as the motor warms up, the grease will melt, and lubrication carried to the motor shaft provide sufficient lubrication. Tri-Flow oil will not melt at any temperature. You can’t go wrong using Singer brand motor grease.
I’m not endorsing one brand of oil or grease, I’m just telling you what I use. Just be sure that any product you use to lubricate your sewing machine is recommended for that specific purpose.