1st gear slipping

[cb750k4]

Okay guys, I took my bike out today and put about 80 miles on it and about 40 miles into the ride first gear started to slip. If I roll into the thottle real easy I can avoid it but if not is slips out and then back in. I tried to make it happen in 2nd gear and on up to no avail. It is only in first gear. I tried letting out the clutch cable thinking maybe it was to tight and even took the cover off and tried to ajust the screw in the middle as suggested by my mechanic, and still nothing. Was just wondering if anyone else has had this problem before I go ordering a new set of clutch plates only to find out I did not need them. Any help would be great. Thanks.

[Re-run]

When did you last change the oil and what kind was it?

[cb750k4]

Last oil change was about four hundered miles ago when I had my top end seals replaced. Not sure on the oil, whatever the shop put in it.