Backpack sprayers are awesome!! I bought a Chapman years ago and love it. They are so convenient. My only regret is having not bought one sooner. I use mine for spraying 2,4-D on the lawn, grass killer in the flower beds and Glyphosate on the weeds in other areas. I have considered buying a second backpack sprayer just so I don't have to rinse it out so carefully after each use.
Word to the wise. Use reasonable care when handling any strong manmade chemical, especially one that is engineered to kill things. Wear gloves. Wash your hands. Follow the directions, including don't spray in windy conditions.
FYI. The state of California has determined that used motor oil is a carcinogen. You may want to wear neoprene gloves when doing an oil change.
Or you may decide to wear a full hazmat suit when working anywhere near your car. Or just pay some 18 year old patsy at the local lube shop to change your oil and let him take the risk for you.