This is all false.
The viscosity of the oil has nothing to do with the drug dissolved in it. The stuff you had may have been 'thick' for another reason, but it wasnt because of the drug. You cant make a blanket statement about "the oil" used for a test e solution* (not suspension). All diffrenent kinds of oil are used. With varying degrees of viscosity, though Im not sure you can even notice it when injecting (I could be wrong on that, I just dont know).
Further, Ive drawn oil based steroids through insulin syringes before. And injecting with an insulin syringe is no problem either. The physics of the the very small barrel and very small needle allow this to happen. Diameter of the plunger, length of the needle (most importantly), and guage all effect the flow. Obviously injecting is much easier than drawing, but it can be done.
The speed of the oil coming out of a 23g is definitely not the problem here. First of all, a smaller guage will force a slower injection, which is desirable. Injecting too fast can create discomfort. The maximum speed the oil is going to come out of a 29g is a lot slower than that of an 18g. So I think your theory is plain wrong on that. You can test if it youd like.
Take a 3ml barrel. Get a 18g or 20g and time how long it takes to squirt 3ml out while pushing as hard as you can. Then take a 25g attached to a 3ml barrel and try again. See which one fills up the bowl faster. Pushing as hard as possible (a bit imprecise but I think there will be a diffrence.