All seems to be quiet on this front lately (only me posting so hope I am not overstaying my welcome). I thought it might be worth trying to pull together all of the various contributions made on this topic in the form of a table (see attached). This comprehensively lists, I think, all the various factors raised for consideration by your various posts. It then lays out alongside each one, features of Multigrades and Monogrades for comparison. No real conclusion defining which is ultimately best of course but depending on your primary concern, may provide a useful summary of the arguments for and against, all on one page.
I had an email exchange (see footnote below) then a chat with the Castrol Classics tech guy who was extremely helpful. He kindly read my table over and offered a few comments but did not take exception to anything that is included here. He did point out that while monos do contain some detergent, its very minimal compared to multis.
I hope this table may prove useful, especially to newcomers wanting a quick intro to the debate.
Initial Email Reply from Castrol:
The current Castrol recommendation is Castrol Classic XXL40, however as you have the in-line filter then you will probably benefit from the Classic XL20w/50. The one thing to take into account is that with roller mains the chewing action of the rollers on the multi-grade will reduce it's viscosity over time, so change the oil every 1000 miles and you will be fine. Monogrades (XXL40) are not affected in this way and retain their viscosity even when due for change.