Oil in the intake can only come from PCV system
So thats what you need to check
PCV valve is a negative valve, it CLOSES with high vacuum, and OPENS with low vacuum
At idle vacuum is high, so PCV Valve should be closed or it will suck in oil vapor from valve cover into the intake
Other thing is oil quality
All piston engines have "Blow-by", anytime a cylinder fires HOT gases will "blow-by" the rings, hence the name, lol
These HOT gases WILL vaporize some of the oil coating the cylinder walls and piston sides, this is where ALL the oil vapor in an engine comes from, blow-by, nothing else in an engine gets anywhere near HOT enough to vaporize oil
A good brand of oil will produce less vapor, cheap brand more vapor
Most of the oil vapor will condense inside the crank case but because the vapor is HOT it also Rises(hot air rises)
And it will travel up the oil drain holes in the head and get into the valve cover areas
This is where the PCV valve and Breather hose are
And this is how the oil vapor gets into the intake, thru the PCV valve and/or the Breather hose
You can add a Catch Can to the PCV system, not at all hard to do and a few NEW cars need that, lol
So an older engine can use it as well