B757 First Officer reported the flight deviated left of course on an oceanic route to avoid wake turbulence; but later realized deviations were only authorized to the right.

Narrative

Westbound on NAT C at FL350 in Shanwick Oceanic our offset was zero. When we encountered minor wake from the preceding aircraft 1000 ft above. The Captain who was pilot flying announced to me that he was going to offset L1 to get out of the turbulence. We applied L1; verified and executed in the route page. At the time of execution I was thinking that something is not quite correct but didn't think too much of it after. Eventually we changed crew for break; the Captain was going for his break and the Relief Pilot was coming back; and a changeover briefing was accomplished including we were offset L1. A [few minutes later] Gander queried us via voice saying that they show us offset L1. We responded affirmative and there was no further communication. It was at this time that the Relief Pilot (who was now the PF) realized and pointed out that he didn't think we were allowed to offset Left. They confirmed by looking in the QRG/ FOM that Left offsets were not authorized and switched immediately to R1.