Depending on amount of leak, you may want to get fixed. If you are smelling it, can you see wetness anywhere? Oil leaks can lead to fire if enough leaks onto hot exhaust. I would verify how bad. If to much I would fix it and not chance a fire.
Just remember it is not generally problem with plug failing, todays Iridium spark plugs go a long time; it is more about making sure you can remove them when they do need replacing. I would rather replace early [60-90k] rather than chance stripping threads in head.
The biggest issue in my opinion is plug seizing in head due to carbon buildup on threads and not being able to easily remove it.
This is same type of problem one gets from never rotating tires. Wheels get seized [corroded onto hub] and extremely difficult to remove if you ever get flat and want...
I would not be to concerned with that if all else looks ok. Some service departments are way over zealous on selling extras during visit. Their used to be a shop here in Twin Cities that sold every brake job new backing plates....may not hurt to ask about it but I don't think it is a big deal.
Some of the fluids sold simply don't meet the spec stated on jug, says 20 below but isn't. Saves the vendor money by selling cheap crap.
You will just have to get all the old stuff out and quality new in.
Considering the picture....it appears to be grease from axle boot? Get a can of brake cleaner and some rags and clean it up and see if it reappears right away. It may not be an issue, they all seem to seep some after a few years.