This should at least apply to the early model JDMs until 1996 and even then not too many things changed but I take no responsibility for the accuracy of this info even though most has been sourced from the Service Manual itself.
This is basically my own mini-guide and check list for my soon to be landed Subaru Impreza WRX Type RA
Engine Oil (4.5 Litres):
10W30 Royal Purple (Amsoil is not available locally) will be my preference based on the Vancouver, Canada climate.
Subaru recommends between the range of 5W30 to 10W40 depending on driving conditions and climate.
The service manual does not recommend a specific oil but for colder conditions you should use 5W30 and for extremely hot conditions use 10W40
Oil Filter:
The good folks at NASIOC and also Type RA owners club assure me that basically any WRX or STi oil filter will bit and do the job just fine so this will not be an issue. Just pick your favorite brand and go with it.
*Important note about the coolant
Subaru recommends that you only use genuine Subaru Coolant to avoid corroding or rusting the engine because many coolants contain chemicals and/or additives that may harm our precious EJ20 engines. They don't force you to go with genuine Subaru oils so I'm sure there is a damn good reason we should heed their advice.
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I've attached a list of Subaru's recommended oils and also the location of the oil filter & drain plug. Speaking of that the service manual does not point out what side or area of the car to get at the drain plug. Can anyone tell us where this actually is?
For example on my Honda Accord you get to the drain plug by removing the drivers side tire and the transmission is on the passenger side.