I've long thought this to be a problem but had not yet seen it identified as it is with this article/web page. I would say that this problem has a good chance of being very relevant, to quite an extent, in GA aircraft in Australia as we have quite a few old/not so old King Airs flying that pressurise off bleed air ...and it is recirculated. That is just one GA aircraft. And that is before we get on to aircraft like the Dash 8. Dash 8's (high hour ones as well )are operated by companies in Australia now that are not the mainstream major carriers unlike a few years ago.
The engines in any aircraft are of course replaced/rebuilt/overhauled but there are quite a few hours of operation in between these events. The residual small deposits from the bleed air, I would imagine, could still be in the bleed air lines as well and not flushed out. These lines are used every flying hour to duct very hot air before some of it is "conditioned" for pressurisation purposes.
Thanks for the referral of the article Ex-FSO Griffo it might get a few people thinking.