SAE-30 nondetergent oil is what is preferred by many vacuum shop owners on this website and it's who I trust, since they would lose business if they screwed up their customers vacuum cleaners numerous times from using the wrong oil, would they not? Walmart has some of this SAE30 oil. It's a lubricating oil, not a motor oil.
https://www.walmart.com/ip/Super-Tech-Non-Detergent-SAE-30W-Lubricating-Oil-1-qt/52626201
3-in-1 oil is too lightweight for large motors. If you had a blender or a carving knife, sure, but not for a vacuum. I used 3-in-1 only once on an antique cast iron fan I had, and didn't do anything to it. I later rummaged around my garage and used SAE30 small engine oil from my snowblower after the 3-in-1 didn't work, and put a little bit of that down the motor, runs like new now(as new as a motor can after being rusted solid for 50 years). Brought the motor back from the dead. I'd never use 3-in-1 oil on my 1/4 horsepower shop fan, as that motor is way too powerful for that kind of oil.
When you have a big motor that generates a lot of heat and runs at high RPM's, you need thicker oil. Nondetergent oil won't gum up in cold temperatures or when the motor is cold, and it won't make a slimy residue. SAE30 is slightly thicker than SAE20 so it will not just be sloughed out of the motor as easily from G-forces of the motor turning.
As with car motor oil, you will get people that fight over silly little things like that, as you are going against what they have been believing for years or decades. People always stick with what they have been taught from wherever source and won't divert from that. Really, the only way to find out what oil is right, is when you have to replace a blown motor. lol If you have a laser thermometer that can help by measuring temperatures before and after oiling at various parts of the motor. You will always have heat, as it's a motor, but if the temperature drops off significantly then you know you've got a good oil.
I will use that oil from Walmart in my vacuums once I start repairing them, but last year when I oiled my vintage fans, I used SAE30 small engine oil and I did check their temps after 2 hours of running them, and they were virtually ice cold, not at all overheating like they were previously. Took forever for them to lose momentum too when switched off, which is a good thing, as shows they are well oiled.
Good luck!