The ones on the oil cooler can be tough.
My friend's 86 oil cooler just wouldn't let go of those flex lines. Heat, penetrant and a couple of fairly strong dudes could not get those things loose cleanly. Nuts ended up galling the alloy threads on the cooler fittings. Had to have Otto put some new fittings on the cooler. Around $100 maybe? Let's hope you don't have to do this.
So if they're really stubborn, i'd remove the whole gig from the car and cut the nuts in many places to try and save the soft fittings. If the problem is where the brass lines meet the flex lines, the brass threads are a little more sturdy so you can muscle-up on those a little more. Still use care and resort to the cutting if necessary. Those flex lines are only like $30 ea. MUCH cheaper than $325 for a new brass line (or the hassle of finding someone to repair and replace that metric fitting- why is metric still a hassle in the US nowadays?
)
Others have said this and I think so too. WD-40 is no good for these kinds of things. I believe WD stands for Water Dispersant. Liquid Wrench isn't much better. The consensus seems to be PB Bl'aster, Kroil, and Wurth Rost-Off are the ones that actually work.
Good luck