Without coming off as a complete jerk, I am afraid you may not get the answers you are looking for in this forum. I went through the exact process you have just described except I took the spring completely out of the left fork when I did my testing. I tested the following (2.5wt, 5wt, 10wt). My results matched your results, which were if you put thicker weight oil in the left fork leg on a 2009 beta evo, this will not increase compression damping. After (3) different oil types with no change in compression dampening, I put some 20W-50 Castrol motor oil in and there was virtually no effect. My first thought was that I had something wrong with my front forks from the factory, but later confirmed that other Beta owners had the same issue as my bike.
I think the key is in what you said: “However, the passages for the oil are so huge that it provides almost no damping in either direction even if you use thicker oil.”
I have a good riding buddy that is considered our local master motorcycle mechanic (he has gone to all of the schools for suspension work). He took a look at my beta forks and basically concur with what you said. The oil passage holes are too large and need to be welded up and re-drilled. He commented that it is somewhat of a guess for the first few tries before you can get the correct orifice size dialed in.
As a side note:
Don’t be surprised if several guy’s chime into this conversation and tell you that you don’t now what you are talking about. Others will comment that you haven’t spent enough time riding. Heck, I had one guy tell me I didn’t have any business riding motorcycles. At 69, I suspect you have been riding long enough to know what you are talking about.
I really like the Beta’s design, ergonomics, motor, styling, handling, but with back to back testing of other bikes can truly say that the Achilles heal of the Beta is the suspension. I suppose there are commercial reasons for why Beta doesn’t just swallow their pride and put some better forks (Marzocchi) on their bikes and the rear would hurt to be tweaked as well. I am not going to argue which is better, but if you have any “feel” and all, I would suggest riding some of the other brands and come back and tell me how the two compare in the suspension department.
BTW, if I ever get my forks welded and drilled I will let you know how it goes.
I do not think you are a complete jerk. I do not have a Beta to experiment with, yet my Sherco Paioli's are similar.
I do not think there is much if any compression damping, nor are the really designed to. Seems one still must pass oil through these large holes upon compression, and them being large, viscosity change in the range mentioned would make little difference in regular movement. Where the difference would come to play would be in the upper shock range of "sharp edge" impact reaction, or the point to where sharp edge impacts start to literally HURT the riders arms and hands because one cannot push fluid through those holes that quickly. You get a pressure spike!
Now the thicker the oil the greater the spike, because one can only push soo much oil through the holes in a given amout of time, like trying to pound a column of oil through a hole with a hammer! The oil will not compress, so this will create high pressure that will prevent the fork from giving in prematurely its length of travel upon repeated rapid hits, yet any fluid will still hydrospike. (bellyflop in the pool) Water will hurt you! Now try that at 20ft!
This is a limitation of an orface rod fork design, and also why it is not used in rebound on the newer forks, which is more critical for us trials riders. Finding a happy rate for your weight and such is still an option, yet is limited in range. If I am not going fast and not hitting things hard and at a high rate, the standard oil is fine, thinner may give you a bit more shock relief, but you will never push it through those holes any faster than it can go.