As far as the coding issue, I'd strongly consider pulling your reports from a number of sources to confirm that the loan you mention is really reporting as revolving. That is really odd. To start with I'd look at what the reports at Karma (EQ and TU) and creditscore.com (EX) say. Those reports are free and you can get them many times a year.
If all of those confirm the loan has gone to revolving, I'd try pulling WalletHub (TU). Again, free report many times a year.
Finally, I'd look at what annualCreditReport.com says, which is the gold standard for reports -- but you only get it once per year, so I'd wait until you have looked at the others.
If the loan is truly reporting as revolving, I'd reach out to the creditor and see if they have any idea what is going on. A loan reporting as revolving is not ideal, since unlike a true credit card you have no ability to bring it to zero and still keep it open indefinitely.
Regarding your question about the mortgage.... are you asking this because you recently bought a home and are waiting for the mortgage to appear?
Finally, regarding "utilization" there are two kinds: revolving and installment. If the word utilization is used alone, it means revolving and loans are not considered as part of it. If someone is specically refering to installment utilization, it means they are looking at your open installment accounts (typically loans) and seeing how much of the original loan amount is paid off.
Whether one is talking about installment or revolving utilization, the two calculations only look at that type of accounts. No FICO scoring factor lumps revolving and installment accounts together.
When you talk about your scores, I am guessing you mean the FICO 8 Classic model. Is that right? Where do you get the scores you are using for measuring improvement?
The "credit mix" part of your score is probably already being met by the loan that was closed six years ago and I presume is still on your reports. (Can you confirm?)
FICO 8 has an additional scoring factor in the Amounts Owed category, which measures your Installment Utilization. Have no installment utilization (because you have no open loans) is bad for your score, but so is having a very high IU (and when your mortgage reports you will have an IU > 99%).
So the interesting question is: which of those two things is worse? I don't think we have a lot of test data on that, though we should. You'll certainly help us out if you can tell us what happens to your score. In order for this to be a meaningful test, you'll need to make sure your other scoring factors stay stable. For example, if your CC utilization was 15% before the mortgage appears, you want your CC util to be close to 15% after it appears.
We'll also need to know the age of your youngest account now -- i.e. before the mortgage appears.
If your Age of Youngest Account goes from > 12 months (before mortgage appears) to 0-2 months (after), that will cause some scoring pain. But that can always be solved by time (just garden for 11 months and then your AoYA will be > 12 months again).
You may be getting some scoring benefit from an AoYA in the region of 6-11 months, so expect some harm in that regard even if you go from AoYA = 11 months to 1 month.
I pull all three from credit check total, Amex, Citi and Discover score card.
Just something to be aware of here is that the scores you get from CCT, Amex and Discover are all Classic 8 scores, where the one you get from Citi is an EQ BCE 8 score. BCE 8 caps out at 900 verses the 850 of Classic 8, so it's not uncommon for your EQ BCE 8 score to be different, perhaps 20-30 points higher than the Classic EQ 8 score you see from CCT.