Just coming back to this topic.
Reading the comments about copper grease again it triggered corrosion thoughts.
Mixing dis-similar metals is a root cause for corrosion. Copper and aluminium is a classic. However, the grease component will probably mitigate the effect. It has got me thinking that the non copper greases mentioned would help.
Anyway, an annual check and engine covers are good practice.
One thing that surprises me though is how water can stay in the plug recesses. Surely the heat in the engine will dry them out? Unless the car is left out in rain too long. Similar to brakes in some ways. Another reason to drive the car regularly before discs etc get a layer of rust.
