You can go synthetic or you can stick with Chevron Delo 400 15w40... I
used Shell Rotella Synthetic 5w40... great price at WalMart. Both available
With synthetic, with your older diesel engine, I'd go as much as 7500 miles
or 1 year... whichever comes first.
With newer diesel... like 1995 or newer, I'd go up to 10.000 miles or 1
year... whichever comes first. If factory manual say longer, then go longer.
Today's oil is not like the old oil... they last much longer and protect
engine much longer. It is a total waste of money and our time to change oil
every 3000 miles. I would say 5000 miles is minimum.
If you are skeptical about 5000 miles, then do an oil analysis test at say
3000 miles and see what the report say... how much longer you can drive with
that oil. It's a good lesson.