https://eje.bioscientifica.com/downloadpdf/journals/eje/140/5/414.pdf
Well it could be because the TU in castor oil was 250mg/ml and TU in tea seed oil was 125mg/ml but castor oil was 33.9 days +/- 4.9 days vs tea seed oil 20.9 days +/- 6 days. Idk I would think if volume would affect half-life then the higher volume for tea seed would give it a longer half-life.
Discussion
Intramuscularly injected TE is the most widely used
testosterone preparation when depot effects are
required, e.g. for substitution of hypogonadism (1) or
in trials for hormonal male contraception (10). How-
ever, after injection of the commonly administered
dose of 200 or 250 mg, TE has the disadvantage that
it produces supraphysiological serum testosterone
levels during the days immediately following adminis-
tration with a slow decline to the lower limit of normal
following within 10–14 days (11). Patients dislike
these swings in serum testosterone levels, which they
experience as ups and downs in vigour, mood and
sexual activity. Other testosterone esters in clinical use
such as testosterone cypionate or cyclohexanecarboxy-
late show pharmacokinetic profiles almost identical to
that of TE (12, 13), so that these preparations offer
no therapeutic advantage.
Although the current study deals with a much
higher dose of testosterone than administered in
previous studies, TU does not result in supranormal
serum testosterone levels, but in much prolonged
action. Extrapolating from single-dose kinetics it
appears that upon repeated injections of 1000 mg,
injection intervals of 6–10 weeks will be possible. The
prolonged intervals and the normal serum testosterone
levels throughout the injection-free period would be
welcomed by the hypogonadal patient requiring sub-
stitution as well as by the eugonadal male seeking
contraceptive protection.