its probably a syncro. My bII did this, but really bad. So i got a used tranny for 150 bucks and a trans shop (lucked out cuase he only had one, and just got it in...but im sure any decent junkyard will have em')
If anything, id try the heavier oil. It may hurt mpg (personally dont think it would even be noticable...but thats not a fact) and if that doesnt help, start looking for a tranny. And get a repair manual if you dont already, it lists step by step how to remove and replace a tranny. And just drive your truck around until it gets worse and worse until its considably bad (no piont in putting unnessary miles on the new trans) and once its nearing failure, replace the tranny. i would also put a new clutch in while its out.
Im assuming its NOT a clutch issue, seeing how you stated it only does it from first to second. Be sure it doesnt do it in all gears or reverse. If it does, its 99% likley its a clutch issue