It's time to do my first oil change on a Duratec HE engine. As expected, the plugs aren't moving, neither is the sump plug. How much force can I apply to each of them before I start getting into trouble?

It's more the sump plug that I'm concerned about, if I get a breaker bar on it is there a chance I could crack the sump?

[Edited on 27/11/15 by Slimy38]

Spray the base of the spark plugs with Plusgas and leave overnight

Theres always a danger with regards the sump plug but spray with Plusgas aswell first
Shock loading the spanner/key with a rawhide hammer has worked for me in the past

Maybe get a blowtorch on the sump around the plug?

Got them both sorted in the end, soaked overnight in plusgas and then a breaker bar. Not sure what was going on with the plugs, they came out clean and it was only the shorter electrode tip that suggested they were old. I've had rusty plugs come out easier!

And the sump plug finally gave up after getting the car into the air and giving myself leverage. And it didn't even round itself off, one benefit of having six sided sockets.