I have just read this thread with great interest. Acquired a IS250 SE-L just 2 weeks ago, 6 years old (exactly) on 65k miles. I can only contribute from an anecdotal point, not technical.
For the last 6 years I have run Saab vehicles, all 2.0 turbos. A couple of 150bhp Vectors initially and more recently (until 2 weeks ago !) two 2.0T Aeros. Saab recommend Mobil 1 0W-40 engine oil and it is critical to the engine's health by all accounts. Where a Saab engine siezes, it is often due to the strainer in the sump becoming blocked with sludge formed most probably because of too viscous an oil being used. The Saab fraternity are generally not in favour of engine flushing either from what I gleaned from their forum, some members experiencing engine failure after flushing.
I'm no expert and none of what I write is conclusive evidence, however, it makes one think ??
The dealer (independent) I bought my Lexus from gave it a service prior to my collection. The oil is spotlessly clean, but I haven't a clue what it is ? I'll be monitoring consumption closely over coming weeks, I never had to top a Saab up with oil in between services. I don't expect to have to top up my Lexus either, I'll be disappointed if I do.