Hey Dru I agree when I had a problem with the valve covers leaking the oil would come of the back on to the exhaust and cause the smell. especially with synthetic oil. I'm still free of smell.

 

Today I noticed a slight burning rubber smell coming from my engine bay today, smelled like it was coming from the firewall on the drivers side. After searching high and low for a bag or something outta the ordinary I noticed my DIP STICK wasn't fully inserted back into it's little home it was raised about an inch or so above. I pulled it out and noticed the rubber stopper was slightly melted. Cleaned it off and placed it fully back in, the smells now gone. Hopefully someone else will get this lucky too!

 

I have had a burning smell for a few weeks now. I've also notice that my mpg has gone down for the past few weeks though I've been trying to "hyper mile" and get a little better mpg.

I noticed I smelled the burning smell at low MPH, 25 mph, or when I had the sunroof cracked, so that would indicate it is coming from outside and not through the HVAC.. I would kick the HVAC on high and try to pump more of the smell into the cabin, but that didn't seem to make any difference.

Long story short, it is the drivers / lefts side / rear brake caliper sticking.

When pulling up to a stoplight, I noticed the car was stopping, very slightly, on it's own without any brake pedal pressure. When I got to work, I checked all the brake rotors and one was sizzling hot, when flicking water on it, and the rest seemed not to sizzle. The one sizzling smelled burnt and red hot.

Took the wheel off and sure enough that caliper has a busted rubber boot and the caliper was stuck "crooked" and not sliding back and forth like it should be, almost stuck in place.

Hope this helps someone, some day. Check your rotors when you stop. If one is sizzling hot, that may be your brakes cooking you smell.

 

Hey man. I have a '08 g35sx and I get the same smell. And just like everyone else I haven't been able to find a real solution.

A family member is a mechanic and said that it sounds like it could be the AC clutch (he doesn't live near me so he can't actually check it)

I know you said it happens even if the AC is off but I also mentioned that to him and he still leaned towards the AC clutch.
Same with the "ticking" A lot of people hear from the same area under the hood. AC clutch is shot. Still works but it cold be working better and not ticking / smelling.

Hope this helps.