In most cases oil coming out of the dipstick is a sign of either too much blow by from the piston rings or the PCV system not working. I would expect more oil to come out of the valve cover if the cap was left off. Lets check the PCV valve and make sure it is not clogged, if it has not been replaced we should. Let me know how many miles and how it runs. Let me know if it smokes too.
Lets START here
This is a truck I am looking to buy. It is a 350 ci V* motor with 167,000 miles on it. It did smoke some on deceleration after accelerating rapidly. it does not smoke at idle and seems to run well.
If it runs well and does not smoke that much it is most likely just a PCV valve or PCV system problem(clog). The engine has to breath. See if the valve covers are leaking , make sure there are no big oil leaks. Make sure there are no signs of sludge, look inside the valve cover. 350 is a solid engine and can last a long time it treated right.
You could do a compression test if you really want to make sure the engine is strong, pop a few spark plugs too.