Attempting to undo seized bolts - is any particular penetrating oil better than another?
Is it generally better than using WD40?

Yeah - most penetrating oils are better than WD40. WD = Water Displacement. Originally developed for the US Navy, its good for that. Marginal as a penetrant.

Many swear by soda (beverages) with phosphoric acid in it (e.g. CocaCola - the US version anyway ...). Can eat some finishes (paints).

If the bolt is steel, nitric acid can be your friend ... I presume that anyone who uses this approach, knows what they are handling, and how to do so ...

Heat, where convenient to apply (like, torch heat, enough to burn off the oxygen in the oxide), also a friend.

try the GM dealer nearby for # 1052627 'heat valve lubricant' been the standby in the auto industry for years, seen it work wonders...

tim

you're right SDNerd, i overlooked the location...

A mix of half ATF and half denatured alcohol works awesome, best stuff I've ever used. It won't last more than a couple days in a little oiling can since the alcohol evaporates...but it's cheap.

We use penetrene or penetrol on site... Galvanised bolts have a tendancy to sieze up when exposed to water and coal dust due to the weak sulfuric acid created when the two are mixed.

Kroil, best stuff in the world. Hard to find, not a general consumer product, but is available online.
I used to work in aviation, and it's the only thing that would work it's way into severely corroded fasteners.
And yes, any actual penetrating oil is better than WD-40.

Mate, there is a brand called 3in1, (which is owned by WD40 company) and they do a very good penetrating oil. It's in a small black and white rectangular tin with a long plastic spout. It's the biz.

Here is one for the list I learned and use. Soak the fastener with penatrating oil, I have some Knock Er Loose on the shelf, but the trick is after you soak it, take a wrench and tap the fastener for about 10 minutes and add a little more oil and let it sit. Come back and do it again. Patience is key. The vibrations help get the oil to the dry spots. Works for me. Even better is to let it soak over night or lunch with more tapping in between.

Thanks for the info guys.
Bic - is that 3 in 1 general purpose/lubricating oil?

Try Plus Gas: available in uk, used to be brilliant and is still recommended

3 in 1 general purpose oil is not a penetrating oil
jon