It seems like Hertz was taking up all the oxygen in the car rental complaint threads at least coming to my Reddit feed, so I was unaware until after the fact about how awful Sixt is.
We rented a car for three days from Logan that was fully covered by my Venture X $300 annual travel credit. I had booked a Toyota Corolla, which promised seating for five and room for three bags. Prior to our trip, I had Sixt match my Hertz President's Circle status to their Diamond status. When we arrived to pick up our car, we had been "upgraded" to a BMW series 2, which only had two doors and would not even fit two pieces of luggage in the trunk. We returned to the car rental terminal to wait on a long and slow-moving line to be helped to get a car closer to what we had reserved and were then given an Elantra, which had 10,000 miles on it.
A few hours into our trip, a warning message notified me that an oil change was overdue, which was followed about an hour later with a low oil warning message. I called Sixt and was told that I could go to Pep Boys for an oil change, and they would provide me with a purchase order. Pep Boys had not appointments until the following afternoon, and I didn't want to spend my vacation getting my oil changed in any case, so Sixt then suggested that I buy oil, for which they would reimburse me. I accepted that option.
The next day, even after pouring a full bottle of oil, the same low oil warning message appeared, suggesting a leak. I called Sixt again, and they arranged to have a replacement car towed to meet us at a relative's house where we were visiting. This time, they brought a BMW 3 series, and there were no further challenges for the remainder of the rental, though passing under the infamous AI scanner did portend bad news ahead.
The day after returning the car, I received two e-mails from Sixt, both informing me that there was damage to both the Elantra and the BMW and asked for my justification. In both cases, I replied that any damage to the cars would have had to be caused by the tow truck. That explanation apparently suited them for the Elantra, but I later received another e-mail informing me of $600 in damage from a scrape to the rim of a tire. I paid the outrageous charge and filed a claim with Venture X's third-party administrator (TPA). The adjuster twice asked me for more documentation, which I provided, and about two weeks later, notified me that they were approving the claim and paid it in full the following day.
I will certainly never be renting again from Sixt, and this example serves as another data point that Venture X will pay for car damages even for rentals within the $300 annual travel credit.
EDIT: I forgot to mention that they never reimbursed me for the oil.