do rental companys service (oil change) their cars on a regular basis?

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They do, so the cars will be in good condition when it's time to sell them.

They are company assets , so they take card of them. Duh!

Of course not.  They just let their cars go to hell hoping the people that rent them will get stranded and demand a refund. Do you even care what you ask or just want the points?

Big names like Enterprise, Hertz, Avis  do,. They sell rentals puled  from service; all maint. records are readily available. Many get  better care than a lot of personal vehicles. They Have to, many are abused by  renters. 

   A  Rent-a-wreck might Not.

@RR: BS. I Had to clean the rentals I returned, They went Over them, noting Everything. Excess dirt  can look like it was taken off-road; there goes that $250 deposit. . I've rented 5 times from Enterprise, 1991-2020, twice  from U Haul. Never "dirty:". Not even a Yar-Piss, Fiesta or Chevy "Spark".

A TD from you, that   declares himself an  expert in every subject, yet doesn't understand simple interest,  and constantly reposts same racist stuff naans nothing.../activity/questions?show=...

Yes, the cars mileages are tracked by computer and they are pulled for service at the correct intervals. Tires are never patched they are replaced all recalls are done and often the vehicles are pulled out of service until the recall is completed. 

Some rental companies are better than others.

If they didn't they'd be replacing a lot of motors!

Don't count on it.  Most don't even do a good job of keeping their cars clean.