Subaru say every 6,000 miles but I like to do mine more often if I can. Use a Genuine Subaru filter too. These engines will last well but only if you look after the oil.
EJ205 was the basic turbo engine for version 5 and 6. It was also used on the WRX but that car had a higher state of tune. The STi used the EJ207 which had more agressive cams and revised inlet.
In the Applied model code, digit 6 is the trim level. '8' was used for the base turbo model in all markets, so in the UK that was Turbo 2000, in mainland Europe it was the GT and in Japan it was the WRX. Digit 5 gives the market, with 2, 4 or 5 being for Japan (2, 4 or 5 doors), 'K' for Eurpoe RHD and 'L' for Europe LHD. ('Y' was USA if you seen any of them!).
The 17-digit VIN code was not used in Japan when the classis Impreza was built. If your car was a JDM car the VIN would be GF8-xxxxxx. The WRX and STi versions were not sold in the UK until version 7 (New Age 'Bug Eye').