What's the difference between using a thinner oil on the front or just changing the damping rate to achieve the same gF/mm?

Billy: It's not really the same effect. The bigger the gap the more progressive the damping effect is. So in theory like laying the shocks over on a conventional car. But as Stuart has worked out, it's not the top of the shock it's the bottom of the shock on a conventional car

Stuart: When you change the oil neither the setup sheet or the app takes that into account all the gf/mm is leverage pressure as if you had just a coil over spring and measuring that if you look at bills setup its stands out and smacks you in the face billy runs a harder rear oil and rear spring leverage compared to the front but the are both relative to that ends oil rate and are very similar in damping feel which actually makes billy car a very neutral base line but it will predominately drive on the nose more than most the downside to bills setup though a fast sweeper the car could snatch if he is to aggressive as he comes off throtle so bill doesn't until he needs the car to hook in its great if you can do that and is very fast way of driving.
other driving styles bills setup will be horrible for example I would tend to attack a sweeper by turning the car in earlier and re applying the throttle mid corner this way I would be loading the rear tyres up at the point bill would be starting to load the front tyres up I would. then use the setup of the rear of my car ton create the rotation I desired for example lighter diff oil and a slightly different rc and of course a different damper setting for example on the same track pulling the same lap time my front is set to 116.7 (463) and the rear is set to 78.5 (425)

Source:

B. Fletcher