I tried silicone and the diluted detergent as well. No go.

However...
I read about mink oil, ie the stuff I use for shoes and boots, on a furniture website. I have some laying around so I smeared a thick coat on one of the seats, especially where the upper meets the lower, and the squeaking and squawking disappeared! I buffed it off, and it removed that hard shine from the leather, and it actually feels a little softer now. I'm going to give it a second treatment and maybe let it sit overnight.

Amazing how the answer can be sitting on your closet shelf all along. None of those other leather conditioners I bought did a thing.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma