Did you mean the lubricant or the fuel ? May be it is leaking while you are travelling and when parked, you can't discover the location of the leaking which may be a little hole or a defective oil retainer.
oil leak
It can leak oil out from the rear of the intake manifold gasket. The oil sending unit is right behind the distributor and it can leak oil too. There is also a gasket that fits on the bottom of the distributor where it goes into the intake that can leak. And the worse thing it could be is the rear main gasket leaking oil and you would have to remove the transmission to repair that.
Rear main seal.
That depends on where the oil leak is, is it in the motor self or is one of the seals leaking?
The GMC Envoy only has one motor oil filter. The motor oil filter is located on the right hand side of the engine near the oil pan.
replace the seal
10 W-40 motor oil can be used in your Subaru, with a minor oil leak. The heavier oil will not hurt your engine and may slow down the leak.
Sounds like your dipstick isn't going into the pan all the way. it has to slide into the rubber grommet quite aways.
ON the bottom of the motor.
when you drive with an oil leak the oil drips onto the road and makes the road surface slippery for motor cycles and other vehicles.
could be oil burn off from the leak Or during repair, the relative dripped oil on the motor, and the excess is burning off when the motor gets hot!
The oil drain plug on a 2003 GMC Yukon is underneath the motor. It is on the same side of the engine compartment that the battery is on.