That is just trademark law. Normal people make the brand name into a noun instead of an adjective.
Do all of you folks saying “Lexii” have any idea how grating that is to those of us who know any Latin? No Latin word forms its plural with the suffix -ii. The only time you ever see a word ending in ii is when the root of the word ends in i, and the only such word you’ve ever heard of is “radius”. The rule is that you replace the -us ending with -i, so radi-us turns into radi-i. “Lexii”, if it were a Latin word, would be the plural of “Lexius”.
Oh, and as a preëmptive strike, that’s the rule for words ending in -us, not for words ending in -is, even though they’re pronounced very similarly in English. If a Latin word ends in -is, the most common way to make it plural is by replacing the -is with -es.
And yes, there are exceptions to both of these (more so for -is than -us), but one would be well-advised to learn the rules before worrying about the exceptions to the rules.
Yep. Most people would say “He owns a couple of Fords” not “He owns a couple of Ford vehicles.” In this case, the plural of “Lexus” would be “Lexuses.” It sounds really stupid to my ears to tack on a Latin plural onto the brand name of a car. I have a strong dislike of Latin plurals as it is (words like “stadia” and “fora” instead of “stadiums” and “forums” really grate on my ears), but those words all are historically defensible. A new word like “Lexus” entering the English language now, referring to something as modern as a car, even if it is derived from Latin (is it? Sounds like a completely made-up word to me?) should obviously follow the normal English rules for number. By analogy, even the closest real Latin-in-English word, “nexus,” is either pluralized as “nexuses” or “nexus,” not “nexi” or the atrocious “nexii.”
If you put it in quotes and do search you get articles like this:
Toyota’s
What kind of librarian pluralizes with an apostrophe?
One with a sense of humor, I assume.
This being GQ, I’m going to need a cite showing that librarians do, in fact, have those.
A Dutch one.
Jokes about our sense of humor aren’t appreciated.
I have alosways thought the plural of Lexus was Lexus. Just sounds right, like deer or shrimp.
My ear wants me to think that the plural is “Lexes”. This is because “Lexus” is homophonous with “lexis” (a linguistic term meaning “lexicon”), whose plural is “lexes”.
Lex Lotsof
or
Therearenoshortage Lex
(sorry, I am on NyQuil)
Actually, Lexus is already plural. The singular is Lexme.
brilliant 
Luthers!
Toyotas in prom dresses?
I almost hate to ask how you feel about Prii…