If you have a look at the SAE J300 chart, 7.5w isn't mentioned. There isn't a grade between 5w and 10w (so it's a thick 5w or thin 10w), but there now seem to be a few companies that are calling their oils 7.5w and selling them as performance oils, although most of the ones I have seen this morning are API SM, so reduced levels of ZDDP.
Personally, I believe 7.5w is a gimmick at the moment, it doesn't seem to be a grade that big reputable oil companies produce. The companies that seem to sell it now are tuning companies and as they are not oil companies, they won't be making it themselves. That means they are either buying in oil from an oil company and relabelling it, so you end up paying more for the same thing by the time they make some money out of it, or they are getting an oil company to make the oil for them. It's very rare that own brand oils are any good, they are made to a budget and the oil companies want to sell their best products under their own name anyway. It also seems that as the 7.5w oils are sold as performance oils, they are selling them at silly prices. One I have seen in the past was around £85-90 for 4L, and the cheapest I saw when I had a look a minute ago was £14.96 per litre, but that was only a semi. For just over £10, you can get a top quality, well proven, ester synthetic oil from a reputable oil company.
While looking at oil grades earlier, I did see there will be some upcoming changes in the SAE grading system, so a 7.5w might become a proper grade.
Cheers
Tim