2008 Suzuki Swift is losing power

Spark plugs can foul with oil or with fuel if the mixture is incorrect or the fuel is old and stale. If it’s fuel that’s the problem you need to either fix the problem that is throwing the air-fuel ratio out of whack or replace the fuel if its old (and modern ULP doesn’t have a huge shelf life). Experience has also shown that once a spark plug has fouled because of old ULP, no amount of cleaning will bring that plug back to life.

If it’s oil that’s fouling the plug, it’s probably because of blow-by from worn piston rings or valve-stem seals. If that’s the case, the oil level n the dipstick will be falling over time and you may even see evidence of blue-grey smoke from the tailpipe. Without knowing how many kilometres this engine has covered and how it’s been serviced, it’s very difficult to suggest what the cause might be.