I had that happen with my 2001 G too. Are you sure its not the exaust? I thought my smell was coming from the engine but it ended up being the exaust. I talked to a few people about it and they said its from the brand of gas. Some gasses have a higher sulfur content and cause that smelling with some cars. I've never noticed it with any other car I've owned but the G. I've found that Mobil works out pretty well. But after about a year with the car the problem seemed to fix its self and now I can use whatever gas without a problem so I'm not really sure what the issue was. I talked to Infiniti and they seemed to admit that he had seen this problem a lot but really didn't know what it was or how to fix it.
It could also be the engine burning as the other posts said.
Hope this helps,
Jim