OK, well it has been 5000km since I totally cleaned the guts of the motor (sump, timing cover and valve covers off) and catch can, and put the new valve stem seals in and fresh oil. This is what I found.
My opinion is, at 236 000km the Alloytec is a bloody sooty motor. Look at it all, clogging the oil filter, and caught up in the top of the valve cover. I scraped out just one fold on the rag there. It was shiny clean in the oil filler hole where I poked my finger there before, now just awash with soot.
Where does it all come from? It has to be blown past the rings, right? My motor has excellent compression, and still goes like the clappers. Absolutely nothing wrong with. That's just how it is. A lot of soot in the oil.
There is no way the catch can catches most it. The oil filter is doing the lion's share of the work. I emptied out my catch can a few times by opening the tap, but this is the first time I pulled it off. There is not as much soot in there as I thought. Always getting a fair bit of that evil dark brown water, but, which I am glad is not congealing in the motor any more, coating the parts in film.
The catch can does definitely catch everything in the bottom after it enters the can through the inlet, and prevents anything from going back into the plenum. This is the gauze filter on the outlet side. Not much on there.
My oil level only dropped a third of this bottle in 5000km, which is a seppo quart.
That is the cheapest oil, but it is a decent brand. I will try more expensive oil just once when I finish the present carton to see if consumption is reduced or I get less soot. I am not at all worried about consumption, but, it is tiny. Valve stem seals were my problem with that.
Finally, definitely the oil in the sump is cleaner. There used to be soot on the dipstick, but now look at it. Sootless.
PCV valve is clean as a whistle still, and the intake dry as a bone. I had oil going in the intake before cleaning the motor, drilling it out, and putting the catch can in, although the valve wasn't really blocked before I drilled. I guess the holes are just too small. Don't need to block.
Anyway, this is what I reckon. Alloytec motors let a lot past the rings. Catch can doesn't get that much of the crap, but is getting something, so why not. Drilling the holes out is the key to stopping the back pressure/oil in intake problems.
The main thing is, it would be madness to run the motor to 10 000km without changing the oil. How could it cope with that? That itsy bitsy filter is having trouble coping with 5000km.
Or at least wash the filter with gasoline after 5000 if using full synthetic and want to get the value for the extra cost. Get the grit off.
Imagine then running it on the wrong oil.
It is easy to see why using the right oil and changing the oil every 5000 is going to be best. Give the oil pump a bit of love and don't let the soot accumulate throughout the motor.