This is one of the reasons why I always suggest people to run full syntethic Motul along with either OEM or K&N filter to keep on the budget while reliable.
This is how my B13's head looked like when pulled apart after it overheated a few years back. I actually do oil changes every 3000miles and this is how all my heads looked like.
As mentioned above, I'd definitely leave it as it is and run Motul Xcess 5w40 or even 10w30, doing an oil change every 1000-1500miles and in about 3 changes you will get rid of must of the sludge (if not completly) under the valve cover. You could also pull the oil pan out and fully wash it before adding new oil.