I have found obd1(?) reader in my town for $250(US). What can they do to help my 2003 XJ8 4.0 stay in good shape?

 

That price is extremely high in today's market unless it is a unit that does Jaguar specific modules and codes. You need to be able to read OBD codes, and preferably to be able to chart parameters like Lambda sensor readings vs. time. There is a Sticky for suggested units.

 

I bought the Bluetooth code reader by Torque MCR USA very satisfied. Show real time data and reads engine codes and deletes. Plus it will do zero to sixty times and quarter mile time plus other times. Was 40 dollars on eBay works great make sure you get the brand I stated there are many knock offs. There are two apps one is free and the other one has a one time fee of five dollars (I bought that one very happy) highly recommended for the DIY