For an accurate historical record, I'd like to reiterate that Jack (jylee) is correct. The W140 S-class did not go to the V6 engine in 1998 like in the E and C-classes. The W140 continued to use the M104 3.2 liter inline 6 cylinder engine in the S320 until the end of the W140 model run in 1999.
Here is the link from the mbusa site: http://www.mbusa.com/brand/container.jsp?/overview/overview_engine.jsp&yearModelCode=99_S320&class=99_S&menu=6_2
I have mistakenly been stating in many posts that MB went to the V6 in all models starting in the 1998 model year, but I was wrong. For whatever reason, MB decided to continue to use the M104 engine until 1999. They must've had their reason, but I'm not sure what it was.
Some of us (including me) have criticized the M104 engine for its weaknesses, like head gasket oil leaks, but Mercedes apparently never saw the problem or maybe thought they had it fixed, as they used this engine in their most expensive and prestigious chasis until 1999.
You learn something new everyday.