It depends if you have the dual exhaust (in the case of the GT). With the GT there are four o2 sensors. One on each pipe directly before and directly after the converters.
The 1998 Ford Mustang owners manual shows ( 5 quarts ) WITH FILTER CHANGE
There are many online resources for information about the 1998 Ford Mustang. Some places are sites like Wikipedia, who have encyclopedia-style articles about a massive amount of topics.
With engine oil filter change ( 5 quarts ) for the 3.8 liter V6 engine in a 1998 Ford Mustang according to the Owner Guide
With engine oil filter change , the 4.6 L V8 takes : In the 1998 Ford Mustang ( 5 quarts ) In the 1998 Ford Expedition ( 6 quarts ) In the 1998 Ford F-150 ( 6 quarts )
The 2011 Ford Mustang has 24 valves.
The 2003 Ford Mustang has 12 valves.
The 2007 Ford Mustang has 12 valves.
The 2002 Ford Mustang has 12 valves.
The 2012 Ford Mustang has 24 valves.
The 2014 Ford Mustang has 24 valves.
The 2013 Ford Mustang has 24 valves.
The 2005 Ford Mustang has 12 valves.