My response assumes that the renter was negligent. If the loss of oil is attributable to bad maintenance, as mightymoose suggests, I wouldn't pay either. When I read the initial post, I assumed that you were referring to an engine for which you have to mix oil and gas.
If he really promised to pay you back "as soon as he had the money", you have a problem. He apparently doesn't have the money. Secondly, if he files bankruptcy and lists this debt, and if the debts are discharged, you're out of luck. Many people threaten bankruptcy. Not everyone who threatens...
Whoever created the logo is the owner of the copyright. If the copyright owner objects to your use of the logo, you are infringing the copyright. It does not matter that you are a non-profit organization.
I think I found the site where you got the logo, but I'm new and cannot post links...