Penrite always did have some 'odd' viscosity grades in their product slate. The story I heard was they used to sell a lot of these 25W60 and 25W70 oils to the mining industry for underground engines that used to get very hot.
I would have thought a 20W50 would have been thick enough for most 'classic' cars but then again, it can get very hot driving along the Stuart Highway (wonderful, can recommend it!) so maybe something a bit thicker might well be appropriate.
There used to be a guy at Penrite, technical manager I think, a really, really nice guy. Wish I could remember his name now...