Personally I would replace the oil and filter every year or 6,000 miles (whichever comes first) and due to being so cheap I would also replace transmission oil every year as well
The external fuel filter every 2 years
The 944 S2 has 2 x fuel filters
1 is in the fuel tank outlet and can be cleaned easily at same time as replacing canister fuel filter

and the canister filter near the rear axle

As for the air filter fit a K+N and clean it out every year

Spark plugs I use NGK Platinum and simply clean and check gap every year and only replace at 40,000 miles
