Unless you are rebuilding the damper unit you can use car oil in the lowers. Its just for lubrication of the lowers.
This is from the Enduro sSeals instructions for the Fox 36 talas rc2. Not sure what fork you have but you should check out the tech section here…
Enduro Tech
“32) Inject 15cc of multi-viscosity fully
synthetic motor oil into the bottom of
the TALAS side fork leg…
…The multi-viscosity, fully synthetic
motor oil we chose to use is made by
Amzoil®. Any other reputable brand is
acceptable. Note that Fox recommends
damper oil. Damper oil is OK, but does
not stick to the parts as well as the
mulit-viscosity synthetic motor oil.”
So enduro seems to think it is fine, even preferable.
You only need to use fine tuned weights of fork oil in the damper unit itself. Are you rebuilding the damper unit?