No Turbo on the Car Blow-by is about the only source of the Oil in the Intake Manifold.
Even with leaking Intake Valve Stem Seals I am inclined to think the Oil from that would not get into the Manifold.
However, worn Exhaust Valve Guides and worn Valve Stem Seals could cause more Blow By in addition to any Blow By from the Pistons and Cylinders.
(My Father had a gas Car with worn Exhaust Valve Stems/Guides and if he left the Crankcase Breather on the Crankcase Pressure would get high enough it would cause an Oil leak around his Oil Pan. Driving without the Breather on stopped the leak but brought tears to your eyes and you had to drive with the windows down.)
Some have had luck soaking the Pistons/Cylinders with Marvel Mystrey Oil to free up sticking Piston Rings.
Some info from one of the threads:
Marvel MM oil results sticky rings
Origional thread also has + response
http://www.peachparts.com/shopforum/showthread.php?t=258757&highlight=Marvel
Well it loosened the carbon as the compression has increased in the two low cylinders! # 3 was 320 is now 380 and #5 was 160 and is now 340! Drove it about 20 miles hard and fast, there was some carbon coming out the exhaust! Will be taking iton two longer trips this next week: Monday I'll be going over to the coast (SLO) and the SoCa gtg on next Saturday. The trip over the Grapevine should help!
Email response about if it reduced blow-by
The blow by was reduced and also with the nice drive to San Luis Obispo my rpms are lower than they were before! Before 65@3,250 now 2,850, before 70@3,500 now 3,150? and boy does it have power... but the glow plug light is now out (starts ok but the weather is warm. The relay works, so I'll be checking the plugs) and the fuel gauge is dancing (ground?). Oh, well gives me something to do...
Other than an Engine Overhaul there is not too many choices to decrease the Blow By.
It would help if your Camshaft and Fuel Injection Pump Timing were all within spec and if your Fuel Injectors were within spec.