You should be able to get away with mid-grade gasoline. Basically what octane is, is one of the many hydrocarbons in fuel that tries to prevent the fuel from detonating prior to spark, meaning it's detonating due to compression alone. A G35 has a compression ratio of 10.6:1, so engine knock shouldn't occur when using mid-grade fuel, especially if it's not extreme driving conditions. As well it should have a knock sensor, so it should prevent your engine from having problems.
Alternatively, you can look to certain manufacturers of fuel that have an ethanol blended fuel. A slightly higher ethanol blend can still be used in a normal engine, and also contains a higher octane rating. For example, here we have Husky/Mohawk gas, which is a 10% ethanol blended fuel, but the mid-grade is a 91 octane fuel, whereas other gasolines are only an 89 octane mid-grade.
In the end, just be wary of your engine performance. If it starts to seem sluggish and lacking in power, it may be the fact that you're only using mid-grade fuel.