The concept of (relatively minor) silicone ingress beating down the TBN is new to me. Can anyone explain that?
3Toyotas: Do you know which TBN test was used, ASTM 2896 or ASTM 4739? The former will yield a number 1-2 points higher than the latter. The latter is preferred and generally more accurate for used samples.
Also, what is your city driving cycle like? Is it very short (coupla miles) hops without full warmup or is it stop and go over a longer distance/period? The oxidation and nitration numbers were high, so you have some adverse chemical reactions going on there.
Silicon is not super high, but you will want to thoroughly check your air filtration system for leaks. What sort of filter is used in the TRD setup? I know SLC has pretty gritty air quality sometimes, so a filter like an oiled cotton gauze type may not be up to the job. If that's what you have, you my look for a foam sock for it to improve its filtration. Or a better filter altogether.
Ultimately, it's likely a situation of the engine and the driving situation severely beating up the oil and the vehicle owner will be forced to adjust the OCI lower no matter what oil is used. Kinda surprised the HDD didn't do better but it is what it is.
FYI- I did the first 10K run of my life (me or my truck, for that matter) on the previous SL formulation/generation Royal Purple 5W20 (see below) but I have a near perfect driving environment with almost no short hops. Some would say RP is a "lesser" oil but it did fantastic.
RP 10K test
I suppose the lesson in contrasting the two oils is that driving evnviroment and engine type can mean a lot.
