I agree with the comments above regarding using car oil, but you say that after two miles or so it is then fine. So what is it like if you let it warm up thoroughly before moving off? While the wrong oil would have an affect I'm not sure how that would change with distance ridden, which in my mind also rules out problems with the plates and springs as weak springs or worn plates would still be weak and worn when hot. Before ripping it apart I would want to check all the simple things first, what's the clutch cable like; is it sticking? Is there a shaft that goes through the casing to operate the clutch? This could be partially seized and freeing off as the casing warms up, it's a common problem on some older cars. Try to eliminate all the simple stuff first!
