No altitude restrictions on the K because of passengers. When you got above FL 280/290 (and FL 310 was reasonably common) you saved fuel, but went so slowly compared with others up there that ATC were reluctant to approve. Nowadays because of RVSM rules in busy airspace the J is limited to FL280. In free airspace an empty J can get easily get into the high 30s - no pax of course because the cabin goes above 8000 ft above FL322.