Thank you Ed for sharing your endless knowledge to the rest of the world, but even the crickets are tired from reading your post.

To the Author,

Thank you for getting away from your computer and to figure out the densities so others would be able to finish off rough calculations from home. Hrmm, will 4 caster wheels each rated at 750lbs work? I’m hoping so!

Vegetable oil containers can be made from heavy or light weight plastics. Motor oil containers are mostly HDPE and can be various thickness, especially in the extrusion of the bottom of a gallon sized container, which can be up to 1/4 inch thick. In addition, you cannot be sure that the volumes were equal just because the labels said they were. Federal law allows a certain percentage variation in volume measure for various products. Food products can vary from the label by 5%.
Gasoline sold by various companies has various additive, methanol being a major one today. Gasoline can vary up to 1/2 pound per gallon depending on its contents. And of course gas is lighter than oil, it has lower carbon density; crude oil is heaviest, refined oils lighter, volatiles lightest. Think of it this way: methane, with one carbon, is a gas at room temp; octane with eight carbons, is liquid longer, but becomes gas easily. More carbon = more weight = more solid.
A good design would be to obtain a certified scale and a volumetric container that is also lab grade, then weigh the liquids in the same container, cleaning and taring between measurements. Additionally, the standard weights for most liquids are available on- line already and you would not run the risk of misinforming your audience! Good luck with future,”experiments”.