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The biggest issue is that the sliders are aluminum and soft. The oil turns black as wear happens. As the oil picks up aluminum it gets more abrasive. So, change your fork oil regularly.
So, when I built my XS2, just as an experiment I tried el cheapo Walmart 30 weight hydraulic oil. Cheap as chips, and I’ll tell you the truth, it’s been in there for over a year and a half and thousands of miles and I can’t tell any difference in function over my other XS650 which has Maxima 10 weight fork oil in it. You might think it would feel a lot different but it does not. View attachment 180863View attachment 180864View attachment 180865
I use surplus Lidl 5w40 synth motor oil that's proved to be less than great in the engines, so have several litres of multi-purpose fork oil ready and waiting.
As an engine oil it was ok, just that it pegged out at around 1700 miles and I wasn't impressed by it.
However, as a fork oil it's been working fine.
There's a lot of stuff that works. Technology keeps advancing. Lubricants are better than they used to be. I use fork oil. I stick to the same brand, so I can play with the viscosity if I want to. (viscosity is inconsistent between brands)
The most important thing remains keeping the oil clean. That means changing the fork oil with some regularity.
As how the forks don't hold a lot of oil it's fairly cheap to experement with different oil and viscosities.
Try the stock amount of any oil you want. Ride it over various roads, Rough Curvy. Really test it out. Then add 1/2 oz, ride over the same roas to see if you feel a differtence. Add anothe 1/2 oz. Try again.
Now try a differrent oil. And try it. You may find a combo that works for you.
In the 35 mm forks I have I run plain old ATF. To determine the amount. I pull the caps and compress collapse the forks completely. Witth the spring out I pour in about 8 ozs of ATF I have a hand hand vacuum pump. I have a piece of tubing a few feet long. I put one end of tubing on the pump. On the other end I put a zip tie 6 inches from the end. I then slide this end down into the fork. I use the pump to drw out oil. When the oil gets down so it won't draw out I know the level is 6 inches below the tip of the fork tbe.
Lift the bike up, install springs and caps and ride happy.
Adjusting the tension on the spring is important too. Weak springs will let the fork compress more than it should. You want about 20-25 mm of sag for a average riding style. 15 to 20 mm gives a firmer ride and improves handling a vbit for more aggressive riding. Much less and it can get hartsh.
If you have over the 25 mm the ride can be mushy.
Leo
Matt Wiley at Race Tech said we should change the fork oil at every third engine oil change. That's assuming we change the engine oil. Since our fork lowers are not hard anodized, this is doubly important.
As how the forks don't hold a lot of oil it's fairly cheap to experement with different oil and viscosities.
Try the stock amount of any oil you want. Ride it over various roads, Rough Curvy. Really test it out. Then add 1/2 oz, ride over the same roas to see if you feel a differtence. Add anothe 1/2 oz. Try again.
Spring out and fork compressed, I fill mine to 100 mm from the top. I feel no adverse effect and it reduces the dive when I squeeze the brake. That is my happy place. That, and 10 wt Bel-Ray.