I can't find the link right now, but FICO recently released a study stating that it weights installment loans the least when figuring a persons credit. Credit Cards have the highest impact on credit score. I am not saying the 6 months loan will have no impact, but it will be relatively minimal.
On a side note, and I am going to be blunt, if the "only" reason you want to get this loan is to increase your credit score, that is dumb. Go live and enjoy your life, your credit will be fine.
Unfortunately for us, the FICO scoring process is a top secret proprietary formula and the best we mere mortals can do is guess. I have published a 10 part series on rebuilding your credit score for those that have filed bankruptcy under my legal guides. Even though my guides are directed towards those who have filed bankruptcy, this information may help you as well. You will need to scroll through several pages to find these guides, but the link is below. Hope this perspective helps!
The number of types of credit makes a different in your credit score, with increased breadth getting higher scores.
Even if the particular installment loan doesn't do much, *having* something in that category helps.