Which engine oil should I use and what level is recommended

Top 10 Lawn Mower Problems and solutions
10: My mower clumps the grass up when cutting
This is a common problem especially in the spring time. Wet grass builds up under a deck more so when mulching rather than bagging. Mulching grass is great for the lawn but if the grass is wet you are not mulching but rotting the grass. It is recommended to have your blade sharpened and balanced at least once per month or sooner if your lawn has a hardy grass type.
Local lawn shops like www.arcolawn.com offer replacement blades at half price to make it simple. Some shops even offer a trade program so you can have a sharp blade the same day.
Remember mulch when grass is dry and bag grass that is too wet. Never use a dull blade! It is bad for the lawn.

9: Electric start will not turn the engine over
With many brands offering electric start at a low cost to consumers this problem is more and more frequent. One reason electric start cost less now is because of the cost of battery technology. Less expensive batteries make for lower cost but the old adage "you get what you pay for" still comes into play. Many brands have opted for a battery that if maintained will last for about 3 years. However maintaining your lawnmower battery can be as complicated as: trickle charging every 30 days for 24-48 hours then once a season for 48-72 hours. If this is not the track record for your battery It is recommended that you take your battery in to a local shop again like Arco Lawn to have the battery "load tested"
If the battery passes the load test split your system in half by checking to see if the engine will turn over with a direct jump to the starter. MAKE SURE YOU TAKE SAFETY PRECAUTIONS! (like removing the spark plug wire and not having any limbs in the way of moving components).If the engine turns over you know the problem lies between the battery and starter. The common problems with the electric start wiring between the battery and starter include: fuses, switches and corroded wires.
Remember
maintaining your battery gives it longer life, troubleshoot by splitting the system and look for fuses switches and wires as culprits to an electric start problem.

8: Engine stopped running and won't restart
Many problems can occur with a small engine. Many times the simple things are overlooked when checking an engine. Start by checking the fuel followed by checking the oil. If either is out fill to the correct level. Oil running low is the number one reason that engines die out and won't restart (even after putting oil in the engine) This is do to heat build up in the engine and even component wear from heat. You may be surprised to see that your machine has no oil in it. Even if you had it to a local shop like... ahem ...ARCO LAWN for a tune up in the spring the average small engine will use 1/2 ounce per hour of running. This is not a well used engine that is now smoking blue-white smoke as it runs. Walk mower engines hold 18-20 ounces of oil in them. That means that within 20 hours of use the oil is low enough to do damage and at 40 hours the engine is completely out of oil. This does not take into account that as the air filter fills up with dirt and grass it causes the engine to consume more fuel and oil. If your machine has run low on oil it is recommended to take it to a local shop and have them perform a "leak down" test to tell you how bad the engine is damaged and if it is worth investing any more money into.

7: Rope won't pull out
Sometimes related to problem numbers 8 or 10 this can be a tricky one. If the oil is all gone many times the engine has welded itself together inside and needs to be repaired by a small engine shop. You can do this yourself with the right tools and a service manual. I recommend a parts lookup tool like this. You will need to write down the model numbers your machines engine to look it up.
The second most common reason for the engine locking up is grass packed under the deck because of a dull blade trying to mulch wet grass. Again I say it is bad to use a dull blade and don't mulch wet grass.
6: Engine is smoking
An engine that smokes can double as a mosquito killer until the EPA shows up to give you a big fine. Allot can be known about a mower by it's smoke. If you have a black smoke coming out of your mower it is from too much fuel. This problem can be caused by an air filter backed full of dirt or a fuel system that needs service.
If your engines smoke is blue-white and oily it means that oil is getting into your muffler. To find where the oil is coming from a great shop like ??? would use a leak down tester to see if the piston rings were stuck or worn letting oil bypass and get into the muffler but not until after they check the oil level and makeup. If the oil is overfilled and smells of gas then the engine will need a fuel system rebuild (kits are available to do this yourself).

Because of length this has been split into two parts. Please see Top 10 Lawn Mower Problems and Solutions (Part2)

Thank you Arco Lawn
Stihl Inc
and other referenced sites.