We use it here in Canada in our P&W 985s out of overhaul for the reasons previously stated re the rings seating. We run it for 50 hours then flush and change to W100.
It is difficult to come by if you are remote from the big refinery areas, Mixing is not reccommended as are many oil types due foaming, integrity etc. There are operators who run from overhaul with W100 without using 100 at all. There is no manadatory requirement but it may be a warranty requirement from the shop who did the overhaul.
carholme