In summary, it looks like the sled likes a bit thicker oil in this stage of her life. Less blow-by, the oil stays cleaner, mpg delta not noticeable, compression may increase with a thicker oil though. So I don't need a 50, nor even a 40, but I do need a thick 30. What say you all?
I think your experience is a good case study why the current fashion to thinner oils requires prudence with some older engine designs, and some European families in particular. Thin is not "in" for certain engine designs, this being one of them. The bearing, valvetrain, and ringpack architecture contemplated by the engine designers (and often quite a few other parameters including gallery mapping and pump clearances) sometimes dictate other choices.
The Volvo modular family (white block) is a case in point. Even though it is still in production, this is a design first conceived in the 1980's. Even in NA form, the engine outputs approximately 70hp/L, which is quite respectable. In the turbos, such as yours, outputs can reach 120hp/L. The original main bearing journals and ring pack were engineered around a heavier 30 or 40 weight oil. While more recent Honda and other engines are now built for very light weights, this Volvo engine family simply is not. Extreme efficiency was not a prime design goal - durability and mechanical robustness were. If you review its original specifications from the early '90s, the recommendations trend towards a 40 in moderately hotter climates, with never less than a 30 in any climate.
By the late '90s (1999 to be exact), to better comply with CAFE, Volvo undertook a major revision of the ringpack to better accommodate the use of lighter 5w-30 oils. To my knowledge, there have only been very minor revisions to the bearing architecture in this engine, but they can safely and efficiently tolerate a 30 weight oil both at main bearing journals and in valve train under most street conditions.
Your experience with significant consumption with a lighter weight is common, and shared by legions of other white block owners prior to '99. There is nothing new about these findings. Nor is it confined to "high-mileage" situations with them.
Your conclusion that the engine performs optimally on a heavier 30 is spot on. Many pre-'99 turbo owners use either that, or tip into a 40. More than than a heavy 30 is usually not necessary except at the extreme end of the turbo engine range (Rs and modified track engines), and issues of turbo spool up become more pronounced with such oils, affecting overall performance. There is a good reason enthusiast Volvo owners who track like oils such as GC and M1 0w-40.
For the post-'99 engines, good results on a slightly lighter 30 weight are typical, and 5w-30 is the most popular choice for most of them -- as Volvo intended.
The white block is more tolerant of ambients than most, and even in a place like Texas, you should be able to stay in the above ranges.
One lubrication fault that Volvo was guilty of here in N. America was laxity in specifying proper test sequence approvals, particularly as to the turbos. As a result, the field practice tended towards the long-OCI use of "starburst" conventional oils in turbos, which has proven a mistake. There are plenty of dirty and sludged dealer-serviced white blocks to show for it. For a turbo, an A3 synthetic is best for OCIs exceeding 5k miles.
As to particular oils in this engine, I have seen and had good results with the above two, with T6, and with M1 5 and 10w-30. The best consumption results have been with T6 and GC. PP has spiked consumption in every white block we've put it in, but the one UOA I was shown was very good nevertheless.
GC is an older formula now and can be hard to find. So in 2012, if I were to pick one oil for your T5 that you should be happy with, it would be M1 0w-40. Go to a NAPA with an AAA discount and I think it is around $5.50/qt.
ps: 182k is not much for that engine.