Clutch slip under power

[ZMX]

I just bought a 1982 CB750 with 14700 miles on it.  For the first couple of days, full throttle in any gear at any speed resulted in no clutch slip.  Now, at high throttle, the clutch slips and the engine goes to 8000RPM.  I tried adjusting the connection at the handlebars, but when I made it looser, the lever couldn't reach far enough to fully disengage.

Any advice?

[Re-run]

Has the bike done this since you got it? Adjust the clutch back to where it was. Did you change oil recently? If so, what did you use or do you know what is in there?

[ZMX]

Oil was low when I got it, so I added a quart of 10W-30.  I plan to change it soon.  The clutch did not slip at all for the first few days.  Now it slips under power.  I adjusted it as loosely as I could at the handle.

[Re-run]

that is probably the issue. 10-30 seems light anyways. Drain it out and put in a good 10-40 oil or a diesel oil such as rotella t 15-40. it might take some time but that should clear up the issue if it is just oil.
I know oil is a big topic and some will say to use motorcycle specific oil only but these bikes will run thousands of miles without a rebuild on regular oil changed when it should be.

[ZMX]

Thanks for the advice, I'll do this ASAP.

[ZMX]

Got some new 10W-40 oil and a FRAM filter.  I changed the oil and the slip went away completely...for a couple hours.  Now it's back and is basically the same as it was before.

[woodsrider250-2]

You can ruin disks with the wrong oil....Not to say that you or previous owner did.  'Friction modifiers' in modern oils can trash a wet clutch quick.  These additives, in general, are in oils marked with 'energy conserving' on the SAE label....Avoid these.

These bikes also apparently have a flaw in the design of the clutch side of the case.  There is a hole that dumps (an abnormally large amount) oil on the clutch, and anyone serious about performance fills this hole up with no ill effects whatsoever.  I have no pics or instructions for this, but am getting ready to do a clutch job soon, and can maybe provide more information at that time.  My bike, currently, does the exact same thing.  I'm guessing plates will 'look'/spec out fine, but they are getting replaced regardless.

[ZMX]

I don't know about the last guy, but I made sure I got an oil with no friction modifiers.  How long are these clutches supposed to last?  Could the springs be getting weak if the clutch has never been changed?

[woodsrider250-2]

Yes