What causes oil to get on my spark plugs

I'm not a Suzuki expert by any means, but I have been working on cars for 50 years now, so I do have a bit of experience.

In general, there are two ways for oil to get into the combustion chamber and foul the plugs. On top of the engine you have the valves which require oil. That oil is prevented from dripping down the valve stem by valve stem seals. As the car ages, the seal can turn brittle and eventually fail, allowing oil to drip down the length of the valve and onto the piston, where the detonation will splash it everywhere.

The second source of oil is the piston itself, which also, as a moving part, requires oil. At the top of the piston is a set of piston rings that serve two functions; some of them are to keep the expanding gasses from slipping down the sides of the piston and into the crankcase, and others to scrape the oil off of the cylinder walls so as to not leave any in the combustion chamber. When the oil ring fails, it allows oil into the chamber and again you get it all over everything, including the spark plug.

To differentiate between the two failures, you should run a compression test. By the time an oil ring fails, the compression rings will also be significantly worn and you'll see a drop in compression on the affected cylinder. If your cylinders are reasonably uniform in pressure, that would tend to point to the valve stem seals as the source of the oil.

There is one other possibility, that your head gasket has failed. The head gasket seals the oil passages between the block and the head, and if it is leaking it can allow oil to go places it's not supposed to like into the cylinder. Mechanics will typically use something called block check to determine if exhaust gasses are getting into the radiator to check for head gasket failure.

Unfortunately, none of these possibilities is a simple fix. The head gasket is the most straight-forward, as it only involves removing the cylinder head. Valve stem seals can sometimes be replaced without removing the cylinder head, but the recommended way is to take the head off. Finally, if the piston rings require replacing, then you're getting into the territory of an engine rebuild.

Sorry I don't have better news for you. Good luck with your Suzuki.