Jeff, Shortler did not say what year his coach is or what engine he has so I assumed the ISC which was the most common engine installed after 1998. My '98 has the 6CTA like yours but there are very few of these around, I am surprised your '99 has one unless it was actually built in '98. I know the WRV manual states 6,000 miles but the 6CTA is the same engine block as the ISC and a Cummins mechanic told me to use the same figures as the ISC so I am going with 15,000. A moot point since I do less than 6,000 a year anyway.
John and Mary Knight
2015 Ventana 4311, 2015 Cadillac SRX