The difference in thickness is not insignificant.
I started thinking about this when I was putting in RaceTech cartridge emulators, and had to cut a new spacer to compensate for the thickness of the emulator.
I decided to use 1" PVC pipe as a spacer instead of the aluminum tube supplied with the springs (that I installed at the same time) because it's plenty strong, and really cheap. I got a 10' piece for like $5 or something.
Because the PVC takes up more volume, you have to lower the oil level a corosponding amount.
Here's some pics of steel vs PVC spacers:
The difference in thickness is not insignificant.
I measured the difference between the cross section of the steel (stock) and PVC and then devided the inside diameter of the fork tube and divided it by the difference to figure out how much you had to lower the oil level for the length of the spacer.
yea, sounds anal, but I came up with that you should reduce the oil level by 2.654mm (note the excess significant figures) for each cm of spacer. I measured the oil level after installing the emulator. This is important because it takes up space too.
I used a contraption that included a vacum cleaner to get the oil to the right level. It was kind of a hassle to figure out what parts I needed, and get them, but once I set it up I was confident that I'd set the oil level not only the same in both fork legs, but were I intended to.