For the OP, and others, as this is a public forum...
I will agree to disagree on Fork Oil. Here is
a link which discusses Fork Oil properties, in general terms.
The critical property of Fork Oil is that it maintains its viscosity throughout its operating temperature range. Also, Fork Oil is used to absorb shocks, not really as a lubricant. Honda used Dexron ATF for Fork Oil, for many bikes, for many years. Other makers did so, as well, typically in the late 1970's and 1980's model years. Dexron ATF maintains its viscosity, just like Fork Oil; it absorbs shock well, equivalent to 8W Fork Oils; it has potent anti-foaming agents; it is highly detergent, which regular Fork Oils are not; it is less expensive than Fork Oils, typically.
My three bikes were ridden by three people, well over 50,000 miles, combined, using Dexron ATF, without incident. it does work very well, in MC Forks. If it did not work equal to Fork Oil, I, or one of my family members, would have crashed, and we never had an accident caused by front end suspension mishaps. Wife and I have toured, two up, many times, many thousands of miles, on our XII, fully loaded with gear, over some poor, rough, roadways. Never an issue with front suspension. The Progressive Fork Springs made a big difference in the ride and the control, drastically improving it all, running Dexron ATF in the forks.
I scoured the Internet, back in 2006-12 era, to figure out why Honda, and others, used Dexron ATF (note that nobody used any other type of ATF in Front Forks, just
Dexron ATF). What I found was that Dexron ATF was equivalent to 8W Fork Oil.
With regards to Progressive Suspension recommending 10W Fork Oil, I checked their site, and I found nothing on Fork Oil weight recommendations. I found they sell only 5W and 10W Fork Oils, but my local shop offers 5W to 20W, typically in 2W and 5W increments.
I bought, and installed, Progressive Fork Springs in all three of my bikes. Never did I see any Fork Oil viscosity recommendations within the printed instructions, inside the boxes, for any of my three very different bikes back then, nor was there any mention of recommended Fork Oil viscosities on their website. This was across a period of 2007-2012; when I looked, today, I found nothing for Fork Oil Viscosity in the XII-specific listing, nor the general listing. If they had recommended a viscosity as being the best for my particular bike models, I would have gone with it, every time. They know more about it than I ever will, but they never offered any recommendations that I saw.
Every bike is different, every rider is different. Rider's preferential choice.
I would, however, recommend
full synthetic, not a blend, of the Dexron
VI ATF: full synthetic oils last twice as long as the dino versions. If you go with traditional Fork Oils, again, I recommend
full synthetic, for twice the longevity: 20,000+ miles between changes. Brand is buyer's choice.
The very best advice I can offer anyone, is to consult a local,
trusted motorcycle mechanic. Ask them what they think, and what they recommend. Feel free to ask about a specific product, if you have one in mind (Dexron ATF's, for example). if they are worthy of their title, and worthy of working on your bike, they will give you an honest answer. If not, find a better mechanic to work with. Cheers!
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