Nuke, are you sure the clutch started to slip right after you put in the new oil? The CB750K clutches aren't great to start with, and maybe you just recently warped a clutch plate. My clutch went early this year, and I got the APE extra friction discs kit, now the clutch is also like a normal bike's.
As for oil, Nuke, next time go to canadian tire. They sell motorcycle specific oil for way cheaper than any motorbike shop. I'll often use their motomaster motorcycle oil cause i'm sure it's just the Castrol motorcycle oil in a different package, which they also sell.
My take on motorcycle specific oil is that it's designed for the higher temps that a motorbike will see. I don't think it's synthetic. Back when I was into Austin Mini's, I remember reading that synthetic oils break down in gearboxes and since a Mini has it's gearbox in the sump like a motorbike, not to use synthetic oils. I even helped a friend part out a shares car that he bought and the gearbox was full of this creamy slime... don't know if it was synthetic, but it made us think about it.