The good oil ...
With the 60,000 km service due, it's time to change the fork oil again. When ordering some engine oil online some time ago, I threw in a litre bottle of Castrol 10W, knowing this service was approaching, but forgetting for the moment this time around I had decided to try the genuine article (gold standard BMW 11.5W) to see if I could detect any difference. I rued the wasted $17.21.
Today I fronted up to the spares counter for my genuine BMW supplies, trying to act nonchalant as I asked for an air filter, oil filter and a litre of 11.5W fork oil, whilst all the time apprehensive about the price of the latter. The oil was on back order from Germany, but the attendant offered to seek a bottle from the workshop. While waiting, she told me the prices of the air and oil filters: $45 and $35 respectively. The service manager, Josh, eventually arrived with the Holy Grail, willing to offer it on credit to the spares department on the undertaking they would hand deliver a bottle under armed guard when the back order came in. Josh is a nice bloke, but always looks serious despite underlying friendliness. "Changing the fork oil, are you," he enquired politely, knowing that his service department gets minimal business from me (maybe there was a hint of resentment in his voice …).
Funny that the quoted prices are such round numbers (especially as compared with the parts fiches available online - US$36.10 and US$19.57 respectively). I always try to dress down and look old when I go to the BMW parts counter, hoping for a hobo (or at least seniors) discount; sometimes it works ("I'll give you that for only $150" - and I feel pleased that I've got a bargain). I'm not sure if it worked this time because when the fork oil was produced, the stated price was $60 (compared with the fiche price of just $US23). I tried not to grimace, but I think it showed. No further hobo/senior discount was offered. I rode home feeling I'd done the best for my bike. After all, the fork oil works out at only 0.2 cents per kilometre, and my bike would love me for the extravagance.
Then I got to wondering about how much different the BMW oil was to the Castrol stuff, especially given I was happy with Castrol 10W for the last 30,000 km (much better than the original fill, presumably of the original spec 7.5W BMW oil). The oil viscosity label on the bottle means very little apparently, except, perhaps, in relation to the same manufacturer's other fork oils. So I was curious to find out.
The fork oil change will be done another day (it's a meticulous job, so I like to allow plenty of time) but I could do some comparisons in the meantime. First think to note is that the BMW product is a little more strongly coloured – a little more yellow/golden. I suspect this is due to the nano-particle gold additive that justifies its price. Viscosity-wise, I thought I'd do some experimentation, with on-road testing having to await another day.
To measure the relative viscosity of the two oils, I employed the syringe I use for rear drive oil, removing the plunger and filling to the 150 ml mark, then measuring the time taken to drain to the 10 ml mark. For each oil, I repeated this for five measurements at ambient temperature, with good repeatability (standard deviation less than 2% for all measurements).
Given ambient was only 14 deg C today, and fork oil heats with use (calibration testing is normally done at 40 deg C and 100 deg C, I understand) I decided to warm the oil by placing each bottle in a bath of hot water for some time to bring them up to about 40 deg C, and for each oil measuring the temperature at the beginning and end of each test and then taking the average. Given my test rig, was not ISO compliant I determined the units of measure would be centiPanzers (cPz) rather than centiStokes (cSt - who is this bloke Stokes, anyway?). I didn’t try to heat the oil to 100 deg C for more thorough testing – my plastic syringe wouldn’t have handled it and it would have been pretty risky handling boiling oil (in the kitchen …).
(Graph updated from the original posted, where I had inadvertently transposed the axes labels)
With so few data points, it is difficult to draw definitive conclusions, but I'm satisfied that the two oils are not far apart in their viscosity, so if you prefer to pay $0.0006 rather than $0.002 per kilometre for your fork oil, then go Castrol 10W and you won’t be far off the absolute optimum. Otherwise, man up and pay for the gold standard (try as I might, I could not discern those gold nanoparticles, so BMW's oil manufacturer has done a great job concealing them). I’ll change the oil in a few days – I wonder if I’ll be able to discern any difference in service …