Hey again Berni,
I have read in several places that in the tropics it is best to recognize the climate and to build light-weight structures and let them rot/fall apart. This does mean rebuilding every so often, and coming from the midwest like I do, this has bothered me and especially my carpenter father.
I did a little research on used motor oil, and it sure is toxic! According to another topic: The two biggest concerns in motor oil are benzene (un-burned gasoline blown around the piston rings; benzene can dissolve in the oil & not boil out too quickly) and heavy metals from the slippery alloys of the crankshaft etc
https://permies.com/t/4738/homestead/motor-oil-preserve-fence-posts
There appear to be ways to mitigate this toxicity risk, like a varnish over the top (maybe sodium silicate), but the best 'homemade' stuff I can find all involve petroleum products, and the least risky methods use NEW material, like crude oil. I am comfortable with petroleum only as a recycled material, not new.
So this might bring us full circle to building light and cheap, and rebuild in 3-8 years or so. Keep us posted!