I earned it partly by working on some very expensive aluminum motors (and big cast-iron diesels) almost 40 years ago, and this is what I would recommend from dirty-fingered experience rather than theory:

1. Warm up the engine thoroughly the evening before you intend to work on it and spray several doses of penetrating oil (nowadays my preference is "PB Blaster") on the plugs while everything is warm enough to promote the maximum chemical interactions between the penetrating oil and the corrosion.

2. Next morning, when everything is cool, give it another shot of penetrating oil and then get out the tools.

PS. my nom du keyboard is Latin for "Dung Develops" or "Excrement Occurs," or however else you want to translate it.