I had questioned a pressure buildup in mine , too. It was awhile back but I got some good responces from those in the know on here, I just can't direct you where to find it.
On the passenger side I pulled the little breather hose off the intake hose checked by blowing thru it to see if the baffles in the valve cover were clogged. They weren't. A manual said in it somewhere to toss valve covers that got clogged...I really can't see these getting clogged enough to build up that kind of pressure unless your using vasoline intead of motor oil. There wasn't any gunk inside the little hose,,,inside the big hose ,,or any gunk on the trottle body butterfly. Wasn't any gunk in the PVC hose where it goes into the manifold either.
With the very low miles on your engine and without real obvious oil burning,,,like gunk , I'd think it's just plane leaky seal, or valve covers (the corners where a small dab of silicone goes per the manual).
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You didn't by chance use silicone on all surfaces around the valve cover gaskets???
Doing that or overtightening the valve covers will make them leak,,,possibly big time...
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Does the '90 have an ERG valve?????Looking for junk inside the valve is an indicator of wear also.
I really get agrevated with oil leaks. Got spoiled by my old 83 toyota that never leaked a drop. I think it's kinda the nature of this engine to be alittle messy.