The oil comes from your filler cap or the rebreathing tube that runs thru the cylinder head to the oil separator. It could have been spilled during oil changes or could be due to excessive blowby or a clogged rebreathing pathway (oil separator and flame trap). If rebreathing path is clogged you need to attend to it to avoid excessive oil usage and blowing out of expensive seals.

If the residue seems greasy it might be that someone who worked on the car coated the spark plug boots and ends with silicone dielectric grease - some mfrs (I am aware of Nissan and Ford) recommend doing this to limit arcovers and misfires. Some techs might do it as a routine procedure when replacing plugs.

As you pull all the spark plugs their appearance will tell you lots about how the engine has been doing.