oil leak onto glowplugs

[sealica]

I'm fairly certain my valve cover gasket is leaking and I'll be replacing it in the near future. I'm noticing that this oil leak is spilling right on the glowplugs, or so it seems?

Just wondering if this is anything to be worried about? When the valve cover is off I'm certain you should clean the plugs, but in the interim is it a non-issue. It doesn't really seem to be causing any serious issues as the van starts fine, doesn't smoke, runs pretty well. Just would like to understand if oily, grimy, and a bit of rust buildup on/around/near the glow plugs is going to have any effect on their performance really?

Just trying to wrap my head around glow plugs effects on performance a little more. I know they warm up the engine, but at what point is it worth getting new ones? or is it not really part of a tune-up like with spark plugs - completely different function I know ...