confused about synthetic gear oil

[Livolsi, Stephane]

my experience is not what I expected.

late last year, I put Amsoil GL4 (this is the correct stuff as per user's
manual) synthetic gear oil in my tranny (86 5ktq), following a clutch job.
I expected that the synth would stay more fluid during cold temps.  Today
was the first really cold morning all winter -22 C (seriously, it was the
first day that was this cold) and after starting I had the gearbox in
neutral and let the clutch out.  Holy cow, the car actually started creeping
forward, only a cm or so, but still!
IIRC, it's even worse than when I had the non-synth in there

Anyway, am I off base here?  Shouldn't the synth prevent this creeping?

TIA

Stephane