In my experience lump sum vs. payment + extra specified to principal is irrelevant.
In a simple interest loan, you pay off all the interest accrued for the given month with that month's payment... and anything additional either out of your regular payment or in addition to the regular payment, has to be applied to the only balance remaining at that point: namely the principal.
If you're paying multiple times a month instead of one big lump sum, then you might save a fractional amount (we're talking tiny when we're talking 6ish% APR) of interest by specifying that the additional payments go directly to principal as the additional reduction in principal on a simple interest loan (which all reasonable auto loan options are) will make a small difference in the rest of the interest accrued during the month; however, I tend to think it isn't worth it.
Plus there's some advantages if your lender pushes your due date out into the future by making a normal payment: you still get the reduced interest, but you get a much extended tradeline if you let it idle near the end of the payment cycle... and in case if something goes off the rails in your life financially, you can skip a few months when a payment is not due if you absolutely have to. That was a non-trivial benefit for me when I was unemployed for a few months and any cash that didn't have to be spent on rent, was useful being saved.
