With typical dimensions, the canister bottoms out around 4.2mm. It's inserted to some other depth and sometimes gets pushed deeper.
O-ring compression isn't as simple as most people seem to think, as the O-ring is already compressed radially before it's compressed axially by the cover. By my figuring, it is fully square around 3.5mm canister depth.
Adding shims is an OK solution (seems to work) but I don't like the idea of having multiple steel surfaces forming a sealing barrier against the oil pressure, so what I do now is pull out the canister and install a new one with a shim underneath to give a solid 3.5mm depth. Then just use the white O-ring, no shim, no gasket, no worries.
Lots of numbers on my canister web page:
Personally I think 3mm is too shallow, especially for the later canisters with the wider lip. There's nowhere for the O-ring to go, so it pushes the canister in (IMO). I've never seen 3.0 in a BMW bulletin (but I have seen 3.1~3.8).