I've learned the hard($$) way that the 915 cannot be shifted like a modern manual.
I blew out 2nd gear about 12years ago due to powershifting it like a muscle car. You have to pause between 1 and 2 to prevent damage I have found as most others. This is necessary even when all is well in the gearbox.
Over the last 14 years of owning my SC I have been able to form good habits and can shift it pretty quickly without much loss in momentum. I don't drag race it but it is fun to leave a stop and run it up through 1, 2 and 3.
I read somewhere(probably Pelican) that 1st gear was designed only for leaving the pits.
I switched to Kendall gear oil several years ago from Swepco and saw a little increase in shifting smoothness. My bushings have been replaced but I do have a frankenstein built shifter that suits me.