I just changed the timing belt on a '97 Montero SR 3.5L single OHC with 150K+ miles and while I was at it I changed the plugs/wires. Last time I put in Bosch +4 and matching wires. I replaced them with the same. I have oil seeping into the spark plug chambers from the top of the valve cover in all six holes. I found this odd since the vehicle was sitting indoors "cold" for an almost an entire day and there was still some oil actively seeping out. The plugs kept the oil out of the combustion chamber. What could have caused this leak? Could all of the seals inside the cover have failed at once? I have been using full syn 5W30 (in a pinch I'll use 10W30) since I bought the vehicle used in '99 with about 40K miles. I always use a Fram filter and change oil & filter every 3K miles. For about 2 years straight I have been using Mobil1 5W30 but at the last change Wal-Mart was out of stock so I used their house brand in a 10W30. I think this leak is recent. Any insight or suggestions would be welcome. It will be a real pain in the *** to pull up that intake manifold again just to replace those seals.

 

Interesting post regarding the leaks and oil types. There is a lot of controversy regarding whether synthetic oil will cause or enhance oil leaks. Your engine is fairly old so, the six valve cover/spark plug tube seals may just be worn out, but who knows. There could be other problems with your engine. I would definately check/change your PCV valve. If it is not functioning properly your engine crankcase may build up pressure which might push oil into the spark plug tubes.

I am a believer in synthetic oil and Mobil 1 is very good. OTOH Fram filters are not very good filters. Definately overpriced for what you get. Not that they will necessarily cause any harm to your engine, but Purolator filters are much better and cheaper. Wix filters are also great.

The difference between 5W30 and 10W30 is not significant and will not contribute to leaks.

Bosch Plat +4 plugs are crap IMHO. You should stick with NGK double platinums or NGK Iridium plugs. They will last much longer and provide a better spark.

If I were you I would pull that plenum again, replace the valve cover gaskets, put in some good NGK double platinum plugs and replace the PCV valve.

Oil in the plug tubes is bad. Image what a little arc could do. Engine fires are bad.

 

Sounds like a common problem with the 3.0 engines & their valve guide seals.

They've simply aged over time. One way to test if this is your problem is to check if you have smoke upon startup in the morning?

Another test is to let your car warm up then idle for 5-10 minutes (go inside and have a frosty/warm beverage or two). Then go back out and rev the engine to about 3 grand. If you get alot of bluish grey smoke, then you have worn valve guide seals.

The fix is to replace them. Not sure about the 3.5L but w/ the 3.0L it can be done w/out redoing your valves using either the LP air trick or the rope trick to keep your valves from falling in.