B777 Captain discovers that SLOP may not be an internationally accepted procedure when WSJC questions 1 mile offset on a M767 airway.

Narrative

Strategic Lateral Offset Procedures (SLOP). SLOP was originally adopted in oceanic regions to mitigate head-on collision risk and exposure to wake turbulence when operating in non-radar RVSM airspace. Air carrier X strongly endorses the application of SLOP in all oceanic regions; including polar and South America; using the following guidelines: 1) fly on course; or offset 1 or 2 miles right (no ATC clearance required). 2) Offsets left of course are not authorized. 3) Use all available means to determine the best offset (e.g.; radio; visual; TCAS). 4) In the absence of a reason to do otherwise; Air Carrier X procedure is to offset 1 mile right. This position should minimize head-on collision potential while allowing movement either left or right (1 mile) as required to reduce wake turbulence. 5) Apply the chosen offset when entering the non-radar environment or at the oceanic entry point; and return to zero offset at the oceanic exit point or when entering radar coverage. 6) When transiting radar-controlled airspace; request clearance from ATC to maintain the selected offset. If denied; return to centerline until leaving the controlled airspace and re-establish the selected offset. 7)Overtaking airplanes must offset within the confines of this procedure so as to create the least amount of wake turbulence for the airplane being overtaken. 8) Voice; FMC; and CPDLC position reports are to be based on the cleared route; not the coordinates of the offset position. Flight X enroute Singapore Changi on course M767; Position: Non-radar control; 6 minutes south of point TERIX. Position: TERIX crossing XA51Z; FL390; SLOP 1 NM right; passing Air Carrier Y Flight Y FL360; who crossed TERIX at approximately the same time. CPDLC Message Traffic: XA57Z ATC Uplink; Singapore CPDLC 'report back on route.' Response XA57: 'Accepted.' We responded to please verify flight calling; that we were established on route. XB00Z Singapore CPDLC: 'Your last report stated you were deviating 1 NM right of route.' Response: XB08Z 'Accepted.' We responded we were following SLOP procedures; 1-2 NM right; no clearance required. It would appear there is some disagreement between ARTCC's expectations and Air Carrier SOP.