info on this seems quite rare
'Thawing the bike (or even peeing on the hub) solves the problem for a while. The usual way to solve the problem permanently was to get another bike with derailleurs. However there are good reasons to believe that flushing the internals with oil helps. Converting a Nexus hub from grease to oil is also the only sane and economical way to maintain it. People usually use ATF oil since the official Shimano oil is shamelessly overpriced. Once converted to oil, it will have to be maintained at lesser intervals.
Some googling on "IGH, nexus, winter, oil, ATF" should give you some directions.
Until then, remember to shift the bike into an appropriate gear when leaving it. '
amongst things i found on google
ATF is suggested