For whatever it's worth ... I would not bother attempting to activate any loan or mortgage account for downloading. (Not discussing LOC or HELOC accounts here)
An online-connected loan or mortgage account does NOT have a transaction register. All data shown in the account come from whatever information the bank downloads to you, not what is calculated when you record the payment transaction ... if this process works at all.
As a result of being connected, the scheduled payment transaction reminder cannot transfer the amount of principal paid into the (non existent) account register and must use a category, usually something like Loan:Principal, instead. The category seems to vary with the Loan Type you selected when creating the loan account in Quicken.
Effective with Quicken Windows 2018+ you can deactivate an online-connected loan account and regain full control over your transaction register. However, you should still review the Scheduled Reminder (or Memorized Payee List entry) associated with the monthly loan payments to ensure they now transfer Principal to the loan account register and not to a Category.
Quicken cannot accurately determine the exact amount of principal + interest due to a different calculation method used by the bank. That's why you'll always have principal + interest + a small amount of additional principal. And you will always have to adjust the payment transaction to exactly match the numbers shown on the bank's printed statement.
Norman F. Raphael said: Why can't Quicken recognize a simple interest auto loan, and will that feature be added?
sgvlasto said: Agreed Norman - I would think you could tell Quicken that when a payment is made it could compare the current balance of the Loan at the bank, see the reduction of the principal was less than the payment from the bank, and apply the difference of the payment and balance reduction to an interest paid category.