LESS THAN STANDARD SEPARATION BETWEEN ACR AND ANOTHER ACFT. OPERATIONAL ERROR.

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CONTACTED OAK CENTER ON FREQ 134.37 CLBING OUT OF APPROX FL190. CTLR ISSUED CLB CLRNC TO FL290. ASKED IF FL290 WAS SMOOTH AND IF SO; WOULD REQUEST FL290 AS A FINAL CRS ALT. OAK CENTER SAID; 'MAINTAIN FL290.' I READ BACK ON 4 SEPARATE OCCASIONS; 'MAINTAIN FL290.' CLBING OUT OF FL255 OAK CENTER ASKED US TO CHK OUR ALT. I REPLIED; '(OUR IDENT) CLBING OUT OF FL255 FOR FL290.' OAK CENTER SAID; 'NEGATIVE; YOU WERE NOT CLRD TO FL290; BUT FL250. MAINTAIN FL260; TFC 1 O'CLOCK; 5 MI FL260.' TFC WAS IN SIGHT AT ALL TIMES. NO EVASIVE ACTION REQUIRED. I DO BELIEVE THAT OAK CENTER ORIGINALLY CLRD US TO FL290; NOT FL250. IF NOT; THERE WAS A SERIOUS COM ERROR IN LIGHT OF THE FACT THAT I READ BACK 'CLRD TO FL290' 4 TIMES. FILED CRS ALT WAS FL330. NOTE: A SUBSEQUENT TELEPHONE CONVERSATION WITH SUPVR AT OAK CENTER WHO REVIEWED THE ATC TAPES REVEALED THAT THE CTLR DID SAY FL250 AND THAT I READ BACK FL290--NEITHER CTLR OR MYSELF CAUGHT THE ERROR. MORE PRECISE PRONUNCIATION OF THE NUMBERS WOULD HAVE HELPED PREVENT THIS INCIDENT. SUPPLEMENTAL INFORMATION FROM ACN 80703: WE CONFIRMED FL290 ON EVERY RADIO XMISSION AND THE OAK CENTER CTLR HAD ALWAYS SAID FL250. I ASKED WAS 'NINER' USED TO FURTHER CLARIFY 'NINE' AND HE STATED WE HAD SAID CLRLY 'NINER ZERO.'