Another one for spent automotive oil, but I've also used Zepreserve which might be considered an oddball, but I'll endorse it here.
Back to the waste oil though...
One of my employees brought in an heirloom; a dandy old, I mean OLD, little single shot .22lr rifle. He's no gunny like me, just wanted me to identify it. He likely will never shoot the thing, will probably just keep it in the back of a closet forever.
It was all dusty, dirty, had rust here and there, so I told him to leave it at my office for the day for me to look at. Once he was out, I ran it into the breakroom, took it all the way apart, and scrubbed it in the sink with an SOS soap pad and hot water.
Then I put her back together and went in the back to find some oil. I had some Zepreserve, but opted for some waste automotive oil that I'd hoarded in a bucket. A little dab rubbed in and on every metal surface of that rifle, and I was satisfied that it would live out the rest of its life in the back corner of a closet.
I tried to buy it from him, but it wasn't for sale. I would've loved to shoot that thing.
I really oughta keep a full cleaning kit at my office.