Bigmon, not sure who your mechanic was but that is plain wrong. Every 3 to 5k for gas or diesel depending whether dino or synthetic. Diesel has nothing to do with when you change the oil.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)