oil for a 1997 Boxster

[JoeFromPA]

The Caddy you mention has GM's 3.6 liter DI engine, correct? That engine is doing horrible things to all oils. I don't believe even RLI's 5w40 has been able to withstand shearing in it, and that was the one oil that was holding up to the RS4's engine (albeit for moderate run lengths)

As far as TBN and 50% of baseline,that's bunk as far as I know. Almost all oils will fall down by 50% within 5k miles. Amsoil SSO, which has a baseline TBN of 12, doesn't stay at TBN 6 after 5-6k miles....despite being an oil rated for 15k-25k+ miles (by amsoil). Take my comments for whatever they are worth to you, but as far as I know TBN is fine until it starts to:

1. Reach 2.0-2.5 or below
2. Be very low in relation to TAN in a traditional oil (that doesn't apply to oils like RLI, which start with a high TAN from their basestock chemistry)

I understand proprietary datasets - makes alot of sense when developing something privately or for marketing purposes. Doesn't make alot of sense to me when using the data to advise the public. I'm not guessing which you are using yours for, I'm simply making a general observation.

Regarding shear and harm to engine: Toyota under-spec'd an oil and it did harm to the engine. That doesn't have a direct correlation to how a good oil will perform post-shear. BMW M's "famous" castrol 10w60 routinely shears down to a 30 grade in normal use, but it still protects those engines just fine. Other oils also shear routinely and protect fine. I'm not saying it's reassuring; I am observing that shear does not = enhanced engine wear.

If someone had a massive data set of UOAs on a particular engine, they could determine if the incidence of shear corresponded to a linear/exponential increase in particular wear metals. But it'd need to be substantial enough to be worthwhile. My .02