From everything I've read, RLI starts off with a TAN around 2.0-3.0 but barely moves from there.
I'm no expert on TAN, but here's a useful summary I found:
"Total Acid Number (TAN)
TAN is a property typically associated with industrial oils. It is defined as the amount of acid and acid-like material in the oil. Oxidation and nitration resins make up the majority of this material. As the oil is used, acidic components build up in the lubricant causing the TAN number to increase. A high TAN number represents the potential for accelerated rust, corrosion and oxidation and is a signal that the oil should be replaced. Critical TAN numbers are dependant on oil type. Typically R&O and light duty oils have a maximum TAN of 2 while anti-wear and EP oils may have maximum levels of 3 to 4.
TAN - TBN Ratios in Engine Oils
TAN by itself is of limited value in determining oil condition of an engine oil due to the fact that it represents a combination of different chemical characteristics. The acid-like nature of anti-wear additives found in most modern engine oils cause a high initial TAN. Greatest benefit is derived from the TAN by comparing it to the TBN. TAN increases in service as TBN decreases. The point at which these two numbers meet has been indicated as the maximum useful oil change interval for that type of engine in that type of service. Studies have shown that when TAN exceeds TBN, engine wear accelerates at abnormally high rates."
Source:
http://www.usoilcheck.com/oilchek/services/used+oil+analysis/tutorial/tbn+tan.asp
From what I can tell, there are multiple different forms of oil acidity and TAN does not differentiate (organic vs. inorganic, corrosion-causing vs. inert, etc.)
The concern, to me, would be when an engine's TAN number has grown a fair amount from VOA with significant decrease in TBN, and at the same time a concerning increase in dissolved metals (i.e. metals that show up on UOA). I'm making an assumption based upon my elementary chemistry knowledge, but I assume acidity-based engine wear would show up as dissolved metals.
From everything I've read about RLI, it's an excellent choice for your engine for the long haul. So would PP 5w30 though, or many other choices. So for me, it's about what you want out of it.
Joe