Clutch slip Higher gears

I am having the problem of the clutch slipping only in the higher gears when I try to accelerate up a hill. Do you think it is the oil? or a clutch adjustment? I am using a standard 20w50 by Suzuki.

 

How many miles on your bike, has it been sitting or has it been ridden continuously/recently? Doubtful oil, can't make any other suggestions without more info...

 

28K miles
hs been sitting 9 months, I just got it back on the road. Had the gas in the oil problem. I have changed the oil three times since that issue.
I did try to adjust the clutch cable slightly to stop the red light creap.

 

Ahhh. Thanks. Doubtful the oil you are currently using is the issue, however, the clutch plates' friction material has likely become saturated with the gas/oil mixture, when it really was supposed to live in only oil. "Red light creep" means it was dragging, now it's slipping...my take: Pull the clutch apart (not completely, but enough to get the plate "pack" out) being careful to note the exact order of assembly, and CLEAN everything. Use Brakleen, a spray brake cleaner solvent sold at auto parts stores, and flush the friction pads of all gas/oil residue. Allow to dry THOROUGHLY or use gentle compressed air. Clean everything else you take out too, and check the "plain" plates for scoring or "blueing" due to overheating, the friction pads should also be clean not glazed, worn or scored. If anything is worn out, replace it. If not, then reassemble. Lightly oil all the plates with clean motor oil upon reassembly, and adjust the cable as spelled out in the owner's or service manual. Ride the bike, gently for a while, then see if your problem is gone. At 28K it's gonna be a crapshoot whether it is just gummy from sitting and then getting gas-soaked, or it's new clutch time...sorry to not be more optimistic, but the miles make it a toss-up.