Its not so much about the gauge size just the length of them to be able to get intramusclar.
The gauge size helps with being able to push the oil easier as its a larger hole to flow as the gauge size number decreases
25G works well for oil which is what I usually go for when dealing with oils.
You should be fine with 26G-27G too
I usually use 25G 1" needles, some use 5/8" and the rare occasion somebody might use 1 1/2"
With the 29-32G needle sizes those are usually used for water based things such as hgh, peptides etc as theres not much resistance and flows easily through smaller needles since its not an powerful