The Caterpillar diesel engine manual says you can burn up to 5% used motor oil without affecting the warrantee. It does say to make sure you filter the used oil

The Caterpillar diesel engine manual says you can burn up to 5% used motor oil without affecting the warrantee. It does say to make sure you filter the used oil so as not to gunk up your fuel system. For a large engine that produces many buckets of oil at each change, this is a good way to get rid of the stuff.