same place you pull the dipstick from....you have to use a funnel
Read your owners manual. It will show you where your oil cap is and it will tell you what kind of oil to use...among other things about normal maintenance for your vehicle.
The 1993 through 1995 J-30 takes transmission oil in the power steering unit. I know... it was strange the first time I found this out.
read the dipstick
If you mean "leaded" gas (it it's even available), then the answer is no. It will damage the catalytic converter. Infiniti recommends and octane rating of at least 91. An octane rating of 87 can be used temporarily, as long as you don't accelerate abruptly and only fill the tank partially full.
The 2013 Infiniti JX runs on premium unleaded (recommended).
The 2012 Infiniti QX runs on premium unleaded (recommended).
The 2013 Infiniti M runs on premium unleaded (required).
The 2011 Infiniti EX runs on premium unleaded (recommended).
The 2013 Infiniti FX runs on premium unleaded (required).
The 2013 Infiniti EX runs on premium unleaded (recommended).
The 2014 Infiniti Q70 runs on premium unleaded (required).