The third gear occasionally jumps out and
downwards to neutral when my hand is resting
on the gear knob and exerts a bit of pressure
in the midst of driving.

Could this be weak detent springs or something ?

Or could it be due to using too light a gear oil -
I'm using SAE 85W-90. Maybe I should change
back to standard SAE 90.

does only do it with your hand on the knob if so remove hand from knob except when shifting.

Yes, it's not good to rest the hand there while driving -- only handle it while shifting.

I doubt if the oil is the problem. More likely the shift arm is bent and not moving all the way.

mine does the same thing, right before i want to change to 4th
kinda annouying as i have no where else to rest my hand

oh well, new gearbox

my old box did that outta first, = #### for pulling cooks, i was doin a burnout and it went "!!!!!!BANG!!!!!!!" and it jumped outta first and reved like mad.....whoops.. o well now i have a box that the syncros are nearly rooted.