Industrial oil falls in Class 4, fuels & oils, and depending on what exactly you sell it can also fall in Class 1 or Class 42 or Class 7. Everything below is read from the USPTO's own list of accepted descriptions, not from an opinion about where it belongs.
Searching the USPTO Trademark ID Manual for industrial oil returns 56 accepted descriptions, and most of them sit in Class 4. The manual is the list of identifications the office has already approved, so wording taken from it is pre-approved and settles the class at the same time.
The class follows from the identification you write, so "what class am I in" is really "what exactly do you sell". How the class system works covers the rest.
Fuels, industrial oils, greases, or candles.
38 accepted descriptions in the manual mention industrial oil. For example:
Raw chemicals, adhesives, or additives used in industry, science, farming, or manufacturing.
6 accepted descriptions in the manual mention industrial oil. For example:
Software as a service (SaaS), app development, IT, design, or science and engineering services.
4 accepted descriptions in the manual mention industrial oil. For example:
Machines, power tools, motors, or engines (other than for land vehicles).
2 accepted descriptions in the manual mention industrial oil. For example:
Industrial oil appears in 4 classes because what you make, what it is made of and the service of selling it are separate classes. Pick the descriptions above that match what you actually do.
Making industrial oil is Class 4; a shop or online store that sells it is retail store services (Class 35), and a business that does both files in both.
Fuels, industrial oils and candles. Edible oils are Class 29 and candle holders are Class 21. Essential oils are not in any one class: the manual spreads them across several according to what they are for.
The class decides what a registration covers; it does not decide whether the name is available. Relatedness is judged on the identifications, so a conflict can sit in a class you never looked at: the same name in a different industry.
Check whether your name is clear for industrial oil Searches the live USPTO register, including sound-alike and meaning-alike names. First search $2, then $5.Industrial oil sits in Class 4, fuels & oils. That is where the USPTO's own ID Manual puts it: 38 accepted descriptions mentioning industrial oil fall in Class 4. Depending on exactly what you sell it can also fall in Class 1 or Class 42 or Class 7.
You file in every class you actually use the mark in, and the USPTO charges its fee per class. Selling a product and also running an online store for it is two classes, not one. Filing in classes you do not use is worse than filing in too few: an application has to be supported by real use or a genuine intent to use.
No. The class decides what a registration covers. Availability turns on whether someone already holds a similar mark for related goods, and relatedness is judged on what the identifications say rather than on the class number, so marks in different classes can still conflict.
Not what you sell? Search the class finder for your own wording, or browse all 45 classes.