How much blow by is normal??

[MarkM]

On my 85 300D, when I remove oil fill cap, I see visible exhaust smoke coming out through the hole. When I place my hand over the hole, I feel the pressure build. With 250,000 miles, I expect some play in the valve guides, and I would expect some visible exhaust. Performance of the vehicle is top notch...does not consume oil, runs smooth, etc. I havnt checked compression in a year and a half...probably time. I have always adjusted valve clearances every 15,000 miles, and a couple of years ago I put in new valve stem seals. Regarding blow by, at what point do you start thinking valve job, when play in valve guides results in low compression? when mileage economy goes down due to compression loss? general performance problems?

Thanks,

Mark