Are you referring to the circlip next to the 4th gear? Or the where the shaft passes out of the crankcase (the seal collar?). If it's the latter, it really shouldn't be stuck unless some corrosion has set up between it and the shaft. Although I am not the biggest fan of PB Blaster - it will work if you soak it good and set it up so that the penetrant will pool and work its way in. My favorite penetrant is a product called "Free All" but A) it's pricey - $15/can, 2 can minimum and B) once you use it you can start a count down for how many days the spray valve will last because it is a very "aggressive" penetrant. But it works like a hot knife in butter! Whichever you use the whole secret to penetrating oils is patience... spray, wait, spray, wait, tap with soft faced hammer, spray, wait, tap, repeat until it gives up - it will unless it's galvanic corrosion. If it's galvanic, it amounts to being lightly "welded". In THAT case see if you can get access to a 10 ton or better press - set up your countershaft very carefully to protect the end surfaces and just force it out!