With my 2007 G35 S, I was initially going 7500 with synthetic oil. Then I started going to my current mechanic who runs his own repair shop. He was previously an Infiniti master technician for 10 years. He said that the high compression engines in the G35 are designed to burn oil, so it's better to go with 3750 using regular oil, since you don't need the synthetic to last as long as would be going to 7500. I was at 7500 when I first went to see him, and he showed me on the dipstick that I down almost 2 quarts as he predicted. He said that if I wanted to stay at 7500, I should check oil levels and add as needed. But he really recommended 3750 with regular oil as the better choice for the G35 engine.
In the JX, the same engine is not tuned to be as highly compressed, therefore the lower horsepower. It might be OK to go to 7500, but in that case I would use synthetic, which lasts longer, and also check the dipstick intermittently and add as needed. That being said, we bought the prepaid Elite Maintenance Plan (discussed in another thread) which includes oil changes at 3750, so that's what we'll be doing with the JX.