No flame, just some reasons to continue to care for that high mile NB.
The alternate choice in oil is your choice, of course, and Rotella 15w-40 is a great non-synthetic oil. But, its not the oil spec'ed for your engine design, as you know. Non-synthetic oils just don't perform at the temps that are possible in the turbo and piston ring areas as well as the piston cooling oil sprays. I think you would save more $$ (which you indicate is the reason for the switch) by running Mobil 1 TDT 5w-40 and 15/20K change intervals, which many, many guys have shown by their used oil analysis to still hold up very well. Changing less expensive oil (and filter?)more often doesn't add up. I've been getting M1 for 5.79/qt with a $12 rebate on 5 qts. Two rebates per household, so 10 qts at a time and they have run this promo twice a year. That nets out to $3.49/qt. Fewer changes (although I still change at 10K interval) fewer filters, less work for me on the two TDIs. So, even if you paid about $2/qt for Rotella, it would be very minimal difference, if any, including filter costs.
I don't think what you're doing is all that big of an issue, just don't see much savings consider that you're getting/using a lesser product.
Nice report on the NB! Getting very close to the big 500K!!