Blowby is combustion gas leaking by the compression rings. If you have a lot of "smoke" comming from the oil filler cap, it's can be bad proportional to the amount. An engine, gas or diesel, when put under power, has more combustion chamber pressure, and will produce more blowby than setting at idle. While it's hard to say what's to much, it is safe to say that if your going to work an engine, that is pull trailers or drive up long steep hills, you are at risk of burning the ring lands off a piston and having an engine failure. The next time you do that blowby test at the oil filler cap, have someone go put the car in drive and powerbrake it compair that blowby to the no-power test. Also, if blowby is a smooth flow, it's telling you that all the rings are worn a little. If you see intermitent (puffs) it means you may have a broken ring or damaged piston. Also, I do a quick comp. test by holding the fuel stop to off while having someone crank (start) the car. If I hear an even, consistent sound as the engine turns over, it's a sign the cyl's are of equal compression. If every 5th has a free sound, i'm looking for a compression issue, either valve or piston/rings. Hope this helps