Although I haven't tried motor oil (and wouldn't), I did immerse my B17 in neatsfoot oil for a good half hour. That was over a year and ~2000 miles ago. It is not falling apart and shows no discernible stretch/deterioration at the rivets. As noted, oil will darken the saddle. Mine is black so that's a non-issue. You won't be able to ride in anything but black biking shorts until the excess oil has wicked to something. Did it work for me? Yes, the leather is still quite supportive but has considerably more flex. I'd suggest riding it without anything (not even the Proofhide) applied for at least 500 miles, preferably 1000+. And like has been stated, plain old water will aid the break-in process. Plenty of BF posters have commented that their Brooks saddles have benefited from a good ride in the rain, whether it was intentional or not. I did try water but must not have been liberal enough with it though. I applied a wet (not dripping) washcloth over the rear of the seat for ~1/2 hour prior to riding. Proceed with caution and good luck.