From the IFALPA Safety Bulletin june 06:
SLOP Strategic Lateral Offset Procedure
1. Crews should be aware of this procedure for use in
oceanic and remote airspace. SLOP should be a SOP, not a
contingency, and operators should be endorsing the use of
lateral offsets for safety reasons on all oceanic and remote
airspace flights.
2. Crews should be aware of the coast-out to coast-in
operational use of the procedure.
3. Crews should only offset 1 nm or 2 nm RIGHT of
centerline.
NOTE: Operators are reminded that the current SLOP is
designed to mitigate the effects of wake turbulence as well
as to enhance flight safety.
And from the IATA Oceanic Errors Safety Bulletin - Ed. 2, 2006
Strategic Lateral Offset Procedure (SLOP):
With widespread use of GPS, lateral navigation accuracy on the OTS has increased significantly. Ironically,
there has been a corresponding increase in the probability of critical traffic conflicts resulting from large height
deviations. SLOP is designed to enhance flight safety by reintroducing a degree of randomness among aircraft
flying the OTS..
SLOP does not require an ATC clearance. It is not necessary to advise ATC of the offset flown
Crews should only offset 1 nm or 2 nm RIGHT of centerline.
The FMS must be used to program the offset, with LNAV used to maintain the offset course.
Use SLOP procedures for wake turbulence or weather avoidance at 1nm or 2nm intervals RIGHT of track
SLOP is SOP, not a contingency. Operators should endorse the use of lateral offsets for safety reasons in all
oceanic and remote airspace flights where implemented.
The decision to fly an offset should be determined to benefit in randomly distributing traffic. TCAS may be used
to determine the best lateral offset to achieve randomization with other traffic or for wake turbulence avoidance.
Ensure a return to track centre-line prior to Oceanic Exit point.
I'm sure the current versions have more to say but this was the last time I gave an RVSM/MNPS course.