On Wed, 18 May 2016 10:42:34 +0100, Muddymike
wrote:

Do any of you have your own formulas for DIY penetrating oil?

I've used a mix of Dexron (ATF) and white spirit for ages. I've tried
Dexron and Acetone but it wasn't noticeably better, was more expensive
and Acetone is generally less easily available. Unlike White Spirit
it also dissolves many plastics. Dexron II by the gallon is about
£20.

With any penetrating fluid the longer you can leave it on the better.
The white spirit/Dexron mix is cheap enough that you can put a gallon
in a can and immerse things in it for weeks.