Piston design mainly caused a lot of problems with using oil and blowby in the first type and then roller lifter problems can be a problem after 1000,000km the plastic buckets they sit in can split or wear out causing the problem. mine had done just about 220,000km.

I found fine plastic in my oil, I thought it looked like carbon at the time but found out it was plastic from the lifter buckets and caught the lifters just in time, so if you come across this you have to lift the heads and toss the lifter buckets and lifters as well is you best bet, if it has not already buggered the cam up or oil pump and maybe the bearings.
When looking at the oil filter it had very fine plastic coming through the filter that I seen.
I first heard a knocking noise and thought it was going to blow up but it went away and then another day it did the same thing, so I jumped out directly but could not pinpoint where it was coming from, then some days later the same thing but when I jumped out with my stethoscope the bastard stoped making the noise, but the next time I pinpointed it down.

Never heard of such a problem before that, but engine reco mob said it's very common.