The issue is the degree of friction the fluid allows. You could use motor oil back in the 80's because it's a simpler formulation than what they use now. Since then, they have added various friction reducing compounds which work great in the engine, but not so great in a transmission. The syncros need some friction in order to operate properly. If they slip too much, it takes longer and/or more force on the shift lever to get them to do their job of speed matching, perceived as stiff shifting.
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Originally Posted by bowtieguy
i wonder if a 0W30 synthetic would remedy that?
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Synthetics tend to have even more friction reducing compounds in them than dino oil, so I expect it would actually make the problem worse. On the other hand, if the symptoms are temperature dependent as TomO says, an oil with a lower cold weight would probably help.