1998 Jaguar XJ8 P1722 Gear 2 incorrect Ratio

Unfortunately, new oil and filter will most likely not cure your problems. Possible cause could be the J-gate module. It supplies the transmission control module with info on what position the gear selector is in.

Before that, check the mounting bracket where the shift cable attaches to the side of the transmission. These have a tendency of the bolts working loose causing the cable to move around which in turn does not select the correct gear in correlation to what the J-gate is indicating.