People use different doses for various reasons. The older you are, the less endoGH you produce, so older guys are likely to want a higher dose. Young men may do well with 1 or 2iu aday, assuming that they dose in such a way as to still benefit from endo production. I'm assuming, of course, that the goal is to achieve higher than nomal GH levels.
For straight HRT purposes, the clinics often prescribe as little as .75iu a day. The purpose there is just to achieve normal levels.
Regarding the insulin syringe question.......insulin iu's are not the same as GH iu's. There is no syringe for GH iu's, so a conversion is necessary. A ballpark rule of thumb is to use a 6 to 1 ratio insulin/GH iu's.