Bulwnkl is spot on on relevance of TAN in assessing gear train oils conditions .
In engines applications , SoJ has commented no worry on high TAN like 5 , unless its 8ish to 10.
I don't know what TAN is considered high/excessive in your ATF, my hunch is 2.9 TAN shouldn't be worrisome.
Instead I would speculate this 45k km D6 ATF is still serviceable, IDK.
Yep, it may be nice to see what BStone has to say on this.
This LV RL D6 has a very good viscosity retention capability from its virgin KV@40*C 30.7 cSt, KV@100*C of 6.4 cSt.
I would seriously consider a thicker D4 ATF in say, Red Line with its higher KV@40*C of 34 cSt, KV@100*C of 7.5 cSt in any typical ATF applications including yours .
This may possibly enable you an even longer OCI, or longer track time hours whilst possibly lowering further metal wear rates in 57 ppm in Fe and Cu etc.
Btw, the previous 2 OCI are on RL D6 too?
Edit:add notes.