What can I expect to happen as a result of using short spark plugs when the motor specs call for long ones?

Question

I have a 91 quad four engine in a Buick Skylark. I am having to replace plugs every month. I have an oil leak that seems to be coming from a seal on the side and below the plugs. The last tune-up I got made the engine run smooth in park and restored the power but shaky in first at a stop. The mechanic said long plugs are supposed to go in the engine but discovered that short ones were installed. What do you guys think the problem is?

Answer 1

Short spark plugs in a head designed for longer plugs will experience oil fouling and carbonization.
Long plugs in a head designed for short plugs can and often do penetrate the top of the piston (not a good thing)

Best to go with the proper spark plug. Your local AutoZone or other parts store can recommend the correct plug and gap settings for your engine.
The oil leak that you describe is probably a rocker cover gasket leak. Very simple to fix..just put a new gasket on the cover. It should NOT cost very much to do the job as no major disassembly is required.
Go to your local library and check out the 'Flat Rate Manual', look up your car/engine, and see how much time the manual provides for that job.
Multiply the time by the hourly charge from your mechanic to see what a fair labor rate would be.
The gasket price will be freely available at the parts store.

Answer 2

Engine missfire. Not a big problem.
Trying to install long plugs into the head now, may be difficult due to the carbon buildup on the threads in the heads.

As far as the oil leak? Replace your valve cover gasket, and seals.

Answer 3

Short ones are cold plugs. Since they are closer to the cylinder head it transfer more heat. Your Owner's Manual tells you which ones to use.