Well I may have goofed. If the oil sender is electric. When cranking the gauge may stay low because of low available voltage. Or itis electrically locked out when cranking as another member suggested.

Forgot you had a newer model with a serpentine belt. The quickest test i can think of to eliminate that effect. Remove the oil pressure feed wire from the sendor. Hook an ohmeter on the sendor to ground. Cranking the engine should change the ohm reading.

These engines start to make oil pressure as soon as you start cranking. Or a very few seconds later. Or just start the engine and shut it down if no oil pressure is indicated in five seconds.

Tach reading when cranking is subjective at best. Cranking rpm is not all that fast. Or once again low available voltage while cranking may effect it. Or the tach circuit may be electrically disabled as well when cranking. I think your problem is getting narrowed down hopefully.

I do not own a 603 for comparison purposes. At that milage dropping the oil pump drive chain is a real possibility. Few people ever think to check them unless in the area for another issue. They can wear out and break.

I have changed my mind a little. Especially after reading post number twenty nine. You do not want to start the engine if the rod bearings are dry. First the proof or lack of oil pressure must somehow be established when cranking.

Even if it is just loosening off the oil pressure sendor or removimg it and having a stream of oil come out when cranking. Get someone to hold a cup to minumise the mess if it is flowing. The same person of course will inform you it is present. Remember no longer than five seconds probably less in reality the oil output should be present.

If the glow plugs are still out leave them out for the test.