I agree with the above writer, that length of the oil filter is really not of any interest. It has no bearing on anything. But, adding multiple shims IS solving the problem caused by the symptom of erratic factory placement.
What datchew said first was the important thing. Depth of the cannister in the engine block. This was also written up very clearly by Oak in Airmail this January.
The cannister is supposed to be installed at the factory to a depth of 3mm. But subsequent measurements by many people have shown this figure to be all over the place.
My friend's '77 RS measured at 2.9, he was finding his O-rings and shim were really looking mangled after every change. He has started re using the paper gasket, that everybody has now stopped using.
I just measured my '81 G/S this week and found I was at 4.2mm. I have now put in 3 shims.
The recommended compression value for the O-ring should be between 10 and 25%.
A new O-ring should be 4.0mm exactly.
A new shim is 0.30mm
My formula for 3 shims is : 4.0(o-ring) plus 0.9(3 shims) minus 4.2(my depth) equals 0.7 divided by 4.0=0.175 times 100=17.5%
This seems to be within the ball park.
Prior to this there was a risk of low pressure with just one shim as my percentage was 2.5%. I have been lax in reusing my O-rings as they always came out looking like they went in. And as I now realise it was because they weren't being compressed.
Datchew with 3 shims has a compression of 15%, so he could go to 4 shims if he wanted to.