Just bought an MY99, 51,000 miles.
Noticed it sounds rough, sort of choppy, at low revs, especially in 4th and 5th.

Dealer's response was "At 1500 to 2000 RPM in 4th & 5th gears all Imprezas perform in the same manner and I suspect would be close to staling at these speeds"

My last car (import WRX, same age & miles) didn't do this
Could they be right or is it likely to be piston slap?
Would switching to SUL or Optimax help?

I've tried listening to the MP3 Piston Slap but I'm not sure what I'm listening for to compare

 

The dealer is correct, the scooby hates low revs in 4th and 5th. You can't use 5th gear in town unless your going over 40mph IMO. It shudders like hell at 30

 

Thank god for that... i though it was just coz my scoob is a shed.

 

Yep, it's sh1te at 1500rpm in 4th. Forget it, give in, use 3rd