For FICO 8 scoring models, you will get a bump in scores with a mature (24+ months) loan that is paid down significantly. Once the loan is paid off, your scores will drop to what they were before the loan.
With the previous scoring model, installment loans are pretty much ignored if you have at least 1 credit card reporting.
You can build a credit score over 800 with credit cards alone. I wouldn't waste time or money on an installment loan unless you actually need the loan, and if you already have a mortgage reporting, this won't help you anyway.