Magpul Industries Corp., makes an accessory for carrying an oil bottle in some of their rifle grips.
The MIAD/MOE Lube Bottle Core is an internal grip core that holds an included 1/2 ounce reusable lubrication bottle.
It comes standard with the MIAD GEN 1.1 Grip. The Lube Bottle Core also works with the MOE, MOE-K2, and MOE AK Grips.
It retails for $14.95.