Odd smell coming thru vents

[krash3]

Last December I started to notice an occasional oily, gas smell coming thru the vents, only when the car has been idling for 30 mins and if I have just the heater going, no AC. It has happened a couple of times during the summer, but again only if it idled for 30 mins and the AC off.
I have been looking up information and I thought it was the OCC I have installed, but I recently discovered someone posted it could be the carbon build up on the valves being the culprit and time to get the valves cleaned. My car is a 2015 2.5. It has 41,XXX KMS on the clock, so alot of my driving is city, but I try to go for a highway run when I can.
Could it be the OCC causing the smell to come thru, or could it be the possible carbon on the valves, or could it be my lack of miles on the car?
Any help would be appreciated

[G ~ MINI]

Did you have the car sprayed ... rust corrosion treatment.
I use Krown and they and the excess does cook off with the heat of engine and get sucked into the car