oil on plugs, one bank only

[mustgofaster]

I just pulled the plugs on my 93' Lightning & noticed that all of the plugs on the passenger side have oil on them in the area between the threads & the chamfer where it seals against the heads... Below the threads, in the combustion chamber, they are dry, though they all look a little rich. The oil is only on the upper part of the plugs. The drivers side plugs all look good... Tan deposits & dry.
The engine is an F4TE 351 with cast iron GT-40 heads. Overall, the truck has about 250K on it, but I freshened up the engine approx 40Kmi ago. (plugs were installed @ that time) When I freshened it up, I had a fine hone done with new rings, new bearings & the heads were done.

So what would cause this?
Thanks!

[Blackpony]

valve cover gasket leaking?

[scootro]

using that fancy synthetic oil found a leak for ya huh?

[mustgofaster]

Valve covers are tight... no visible leaks.

No synthetic @ this time. I used to Run Amsoil, but after the rebuild, I went back to Dino oil for the break-in & havn't switched back yet. Currently running Valvoline 10W-30.

[lilvega]

Could be an intake gasket leak pulling in oil from the valley. The oil attracts to the hottest areas - exhaust valve and spark plug - and escapes around the threads.
I have had this issue before and when I pulled the intake the gasket was oily on the bottom side.

[mustgofaster]

That makes sense... I have had some problems with the intake leaking in the past. I think I'll pull it this weekend & check it out. If it is leaking, I think it'll be time to take the intake to a machine shop to make sure it is straight. This would be my 3rd intake leak in the last 2 years. The last 2 times, it leaked coolant from the front pass side coolant passage.