Bongo Fury

(https://imgur.com/8MtPzu6)

2 things:
There's some seepage from the rocker cover bolts (yellow) - but what are the 'red' bolts? Also - could the PCV valve be a culprit too?

How do you lot go about cleaning your engines? I've seen lots of pics of sparkling cylinder heads, etc. but the last time I got an engine cleaned was at a car valet who used a steam cleaner (a long time ago). Not very practical when working within the van cabin.

No expert but I used Gunk on a few areas of our diesel engine when MOT said advisory oil leak. I was pretty sure there was none
so I cleaned it using GUNK and as suspected no leak, so advisory was removed it was just the block was painted black and had
23 years of dirt on it.

It does not look that bad from what I can see there, a light misting (are you certain it was not multiple small spillages that was not cleaned and has gathered dirt ? It does not look like a serious leak). But cleaning it and watching where it starts dirtying up might give you a clue. You could rag/brush that down with some fairly liquid + warm water given a bit of effort.

I have heard about blocked PCV valves causing uneven idle speed/revs but don't know an immense amount about that either. On my car I coulld not even find it ! I purchased one for £5.00 and looked everywhere as the engine revision seemed
very early and I could not locate it despite looking for 20 mins. Car still runs ok but I got interested as I have very old vehicles.

Good luck tracking it down.

Those nuts circled red - they're dome nuts. If these are holding the rocker cover down, the same as the others, then I would think they might be at their limit and unable to get any tighter. That won't help.

There's absolutely no oil down the sparkplug well - so I guess it's a mild leak. I'll nip the nuts up, if someone could confirm whether those in red should be the same as the others.