If you are going to drive 4.5K a year, I would not change your oil for a min of two years.
I have certainly considered that, especially looking at the TBN. I just have a slight fear that an EGR cooler or the Oil cooler will leak coolant into the crankcase and it might go unnoticed for too long.
Yes - that is the conundrum, is it not?
In large sump systems, the volume of an OCI makes for expensive events, and therefore annual UOAs are far cheaper and cost effective. When you get into sumps that measure their capacity in gallons, this is a no-brainer easy decision.
In smaller sump systems, it is "iffy" as to where your break-point would be. Most light-duty diesels like the Dmax, PS, ISB are generally at a break even in UOA cost vs. the dino oil OCI. In my Dmax, I can OCI for about the same cost as UOA, but I choose the UOA as a matter of experimentation and fun. I am on my third year of an OCI in my Dmax, and will sample again this fall. AS your 2008 is probably out of warranty, then longer OCIs with UOA confirmation are plausible. That is the decision I've come to for my diesels. I will note that those who use syns and still OCI at "normal" intervals, are wasting HUGE amounts of money; they would be better served to sample before making an OCI decision. It is, without a doubt, cheaper to UOA than dump PAO in the pan every year.
In gasser engines, with yet smaller sumps, it's just flat cheaper to OCI with dino, and perhaps the syn may be a break-even proposition. You'd have to know the exact costs involved. Some UOAs are cheaper than others; some sumps are smaller than others. Too much variation in the possibilities to make a one-size-fits-all comment here.
Regarding your UOA, I'd say no reason not to extend the OCIs. Test annually.