depends on your definition of "a long time" and also on the thickness of the oil, usually UG gear is thicker.
I say 23g is best. 25g not bad but takes a little longer. More than 25g takes too long.
With 25+g it takes a while and you have to apply more pressure and it's awkward to hold a syringe against your ass or delt with one hand for 30 seconds and push the plunger down without wiggling the needle around and **** so I think >25g is stupid.