In reviewing the prior owner's maintenance records, as performed by Lexus, the Lexus dealer was adding an "oil additive" part number 1511, through the 26K mile oil change. The only thing I could find on the Internet with that part number is SPECIAL DUTY OIL POWER BOOSTER 1511 which allegedly "Reduces sludge, gum, and varnish deposits" among other things. Is this a snake-oil additive or...?

Hmmm..... Bardahl is a pretty reputable company, although I can't say I've ever heard of 1511. I guess you could pull the front VC and see what the valve train looks like....
If it's super clean, they may have been on to something...

You can't beat BG products. Dealers use BG for services.

http://www.bgprod.com.au/product/116...e-concentrate/