i've been able to remove OG residue w/ shoe polish (cream color) and alot of elbow grease. i took a soft cloth and rubbed the shoe polish into the exposed leather grip and started scrubbing. took some time, but i was trying to salvage a Head Genuine Calfskin grip so it was worth it.
soap and water won't do it. be careful with the scrubber sponge as it will scuff the smooth leather surface.