WaxOyl is a protective coating for the underside of vehicles. You spray it on with a pressure sprayer or hand pump. It sticks to metal and protects it from rust. It's very much part of the British tradition of DIY vehicle maintenance. When its manufacturers get a web site I'll link to it, but for now I provide this explanation for those from countries that don't have this stuff.
Some people use old lubricant oil for similar purposes.
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