[tedbrother]

I need to increase viscosity of Mineral oil any suggestions? Tried Bentone clay but activator (propylene Carbonate is too expensive to use to activate.

[P]

Will silica thicken it?   -  like fine powdered fumed silica.

[Enthalpy]

For what use? If for instance it's a lubricant, only very few additives are acceptable!

Mix some heavy hydrocarbon, like heating oil or lubrication oil. Diesel oils for marine engines are quite thick.

Mix some lubrication grease.

Many mineral oils dissolve polyethylene and polypropylene, maybe natural rubber. Take them as felt to accelerate the dissolution.

Heat the oil enough that some components polymerize.

Distillate the oil to remove the lightest components.

[wildfyr]

I like the idea of cooking it a little extra to polymerize.

[Intanjir]

Could just use its higher length cousin, petroleum jelly.

[P]

Did the silica not do anything?  Too similar to the Bentonite? How about corn starch?

What is the application of the thickened oil?