If you got a very thick oil, yeah a mix of diesel and oil is good so it would lose on viscosity and would be easier to get it on and into the wood. But today's engine oils are not really that thick and can be used without any mixing with oil of lesser viscosity such as diesel. Nowadays you can find those very thick oils mostly in tanks (military vehicles) and big machines not your everyday family car.
Tradition is to save it and use it as a wood oil so the wood will not decay after some time on the rain. Absorbs really good, doesn't stink or stick...
Onya marx... Get set... Go!