Oil can very well have an effect on it but as mentioned by Chevyfazer, Final Impact and others, when decelerating, slowly down shift accordingly with the slowing speed.. **DO NOT WAIT UNTIL STOPPED ***to go from 6th gear to 1st. Its the way the engine is designed. MY FJR would do the same thing if driven in this manner.
Should you find yourself in this situation, gently letting the clutch out just a little (to get the output shaft gears to mesh with the always driven gears), pull back in the lever should allow you to downshift at a stop. You may have to repeat the clutch pulling. A loose chain will exasperate the problem as well
The WORST thing you can do, DOWNSHIFT ALL AT ONCE AT SPEED (IE 60 MPH). Pull in the clutch as you slow and downshift one gear at a time as your speed slows. Even thou the clutch is pulled in, you still have the transmission shafts spinning super fast. A lot of folks may not realize, even thou the clutch is pulled in, the transmission IS ALWAYS TURNING, it is directly attached to the engine (by gears of course). The clutch is simply allowing the smooth transition between the two shafts/gears(the engine turning shaft and the sprocket/output shaft).
IMHO, I let the engine, depending on conditions/traffic, do most of the stopping with engine braking (down to around 2-3k RPM's). Again, as I slow, downshift ONE GEAR at a time, and let the clutch out, keep slowing.. Keep repeating until stopped and you shouldn't have a problem...
If not using the brakes, (downshifting/engine braking) apply your brakes at least lightly to illuminate your brake light for the guy behind you..
**Letting the clutch lever out after EACH downshift, confirms your in a lower gear...