This idea of adding white spirit to the cleaning oil/extreme winter oil is crazy
I thought so too, when it came up elsewhere it was pointed out that White Spirit isn't even a standardised thing, there's three main types and variations within them.
OTOH - I've been dipping Alfine and Nexus hubs into automatic gearbox oil for years, based on something I read on the internet, and they run smooth, but it wouldn't have made me cry if that hadn't worked and the oil was a tenner instead of £60 for the Shimano stuff.
I use Rohloff oil for my Rohloff hubs, reckoning that it wasn't worth skimping on the cost of the oil for expensive hubs.
I bought a couple of kits to get some spare syringes and tubes, then 250 ml bottles of the cleaning and lubricating oils. And after I finished the 250 ml bottles, I bought litre bottles.
I also now use Rohloff oil in my few remaining Sturmey-Archer hubs. In the Sturmey hubs I use a few drops per hub per month, my last 125 ml bottle of Sturmey-Archer oil lasted for over 5 years, so I decided that with the bulk Rohloff oil available it wasn't worth the bother looking for a cheaper substitute.
I also bought a bulk pack of the Rohloff drain plugs (20 ?), which I also use on the Shimano Nexus 8 premium hubs fitted to six family bikes. On the Rohloff hubs I generally fit new ones at each oil change, the Shimanos get the used ones.
For the
Shimano hubs I use a synthetic gear box oil, easy to find in relatively cheap 1 litre bottles at local motor supply shops.
My logic here is that the official Shimano dunking oil kit costs about as much as I paid for each hub (about 70-80 euros, I bought several when they were available at discount prices at SJS Cycles), and that I have already invalidated any guarantee by drilling the shell for a drain plug and using outboard-motor grease to reinforce the weather resistance in the two accessible outer bearings.
I started using this synthetic oil in 2012, and so far I have not had any problems, so it
probably doesn't damage the plastic parts inside the Shimano hubs. My opinion is that using oil instead of the factory-supplied grease increases the efficiency of these hubs. The drain plug allows me to "top up" the oil to replace seepage through the seals, which is slightly more than the seepage with Rohloffs but still fairly minimal, and so increase the interval between opening the hub for inspection, oil bath and regreasing the outer bearings, which I generally do about every two years or so.