Best wood preservative I have found is a mix of RV antifreeze (propylene glycol) and Boric Acid. Soak good..
Or use borate rods inserted on 12" centers (depending on the size of the post)- that is a safer way to prevent rot and insects.
We have borate and copper sulfate rods that we put into holes bored in our log house in areas where wetness is prevalent (due to the guy who built it cheaping out on the overhangs), we bore a hole, put in the rod, and cap with a plug of SYP (same as the logs) to prevent fungal rot and insect infestation. The rods slowly diffuse into the wood when it is wet killing fungi and bugs.
The best price for the size and qty we needed was at
www.coppercare.com.
BTW, as to the motor oil safety issue, it says right on the bottles that it is a known carcinogen, and having it run through a hot engine where petrochemicals are being incompletely combusted can't make it any healthier, and it is known that used oil has all manner of toxic things like heavy metals, PCBs, etc.
In the no so distant past bunches of hazardous things were released into the environment, a lot of which no one knew (or admitted to knowing) were as bad as they ended up being.
Remember DDT, Agent Orange, etc, anyone.
At the campgrounds where we camped at summers while growing up they used to drive through and spray the whole area with pesticides once a week to kill any mosquitoes because people complained about them.
Typically, we were outside playing while they were doing it and no one ever told us to go inside to avoid the spray or wash our clothes or other precautions.
Yeah we're still alive (well most of us), but who knows what the exposures did to us? I ended having a rare bone cancer at age 19, with no family history of it.
We now live on property that was a fruit orchard 50-100 years ago, and a good friend who is an environmental engineer (and does environmental safety studies) recommended that (between the oils they used to spray on the fruit trees and the superfund site 3 miles away from us, where a town highway dept employee had stored barrels of the oil (not asphalt) they used to spray on the dirt roads to keep dust down 30 years ago, both of which ended up leaching heavy metals and PCBs into the ground water), we should only raise veggies in soil brought here from somewhere else, and put into raised beds to sequester it from the native dirt here to avoid heavy metal poisoning.