Northstar pw with comet axd 3030 pump. where is pressure relief valve located. I am only getting half of normal pressure and read that this might be stuck open

probably has a relief valve in the pump head that prevents over pressure from damaging the pump
3000 psi is extreme for a pressure washer and will remove paint from a vehicle , blow out gaskets on engines , remove cement surface , peel timber surfaces

30 weight non detergent works in most pressure washer pumps. available at most auto parts stores.

junk axial wobble plate pump. Usually axial pumps are not worth the time to try to fix them. This one has a 5 year warranty , so they think they have a pretty good one. ( I would never buy one) If you want to try to fix it, I would check the check valves first. Take them out and see if some are broken or not sealing properly. There is a valve kit available.
The other likely problem is your unloader valve. If the seal is shot or the valve stuck open the pump will not build pressure.

parts diagram http://www.ppe-pressure-washer-parts.com/files/1961581/uploaded/axd-PUMP%20BREAKDOWN.pdf

check your check valves.
or one of the wobble plate pistons is not working right ( broken, broken spring etc)

this is a piston seal issue. You can get a kit to rebuild the pump.

These AXD pumps usually do not require a rebuilding process. What happens is the unloader / bypass valve gets stuck and you do not get any output pressure. What you can do is tip you pressure washer so that the input and output fittings are vertical and actually dump oil into the water supply hole. (3 oz at least) Slowly pull the recoil (ign off) and add a bit more oil and let it sit overnight. Then the next day once again pull the recoil through and let it sit another 10 minutes. Then connect all hoses (Purge the air in the garden hose) BEFORE connecting the main water supply and start the unit then running at MAX SPEED rapidly fan the wand trigger. What your doing is cycling the unloader valve. Do not do this for more than 2 minutes if the pressure does not suddenly start. IF the pressure builds continue at lease another 30 rapid cycles of the wand trigger.
I lost count on just how many pressure washers I have brought back to a good working service life by just doing the above. I do know it is a substantial number.
Give this a try before you go off on the need to rebuild the pump avenue.

You should use what the pump manufacturer recommends. Typically a 30wt non-detergent (non foaming) oil. Do not over fill. If there is a sight glass half way is normal. Or if it has a dip stick follow the mark there.
Here is a great link for pressure washer parts and info
This link goes right to oil page:
http://pressure-washer-parts.com/shopdisplayproducts.asp?Search=Yes

Normally, it is one of three things.

1.) The high pressure seals are bad

2.) There is debris in the check valves

3.) the unloader is stuck in by-pass.

aftermarket oil will be fine just use same grade of oil,all manufacturers want that extra kick,

I dont know which comet pump you have, but it is just easier to get a new pump from one of the pressure washer companies on the internet and they sell them on ebay. You can also probably get a better pump that will fit your engine as well. Also, have you tried just changing the oil and not fill it too full? On my 5000 lb washer, if i fill the oil too high when I change the oil, it will foam some. Hope this helps you, Rick