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Old 08-23-2011, 01:47 PM   #1
Scooby Specialist
Member#: 226211
Join Date: Oct 2009
Location: Europe
2002 impreza WRX JDM

Default 5w40 VS 10W40 synthetic

Hi everyone!

I have rebuilt ej205 with stock internals.
cranck is stock in pair with acl aluminum std size bearings.
clearances are: main: 0.025 - 0.03mm
rod: 0.035 - 0.04mm

now I'm breaking in my engine, but want to know about oil, I will use after. we know that good 40 grade oil is good for that clearances, and I have choice between mobil 5w40 and mobil delvac 10w40 synth. (I work in the company which is official importer of Mobil 1 in my country)
the car is DD, with some autocross or track days max once a month. I have short trips everyday, like 5-10 miles (my work is too close from my home)

one case is pressure on hot idle, but: for more adequate choosing, on what rpm should ej205 hit bypass valve pressure 6.2 bar (about 80psi)?
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bako is offline   Reply With Quote
Old 08-23-2011, 06:04 PM   #2
Scooby Specialist
Member#: 43950
Join Date: Sep 2003
Chapter/Region: MWSOC
Location: Minneapolis, MN
07 WRX LTD Wagon
Satin White Pearl


It sort of depends on your location (temperature). The 5w40 should be better for cold starts, but it basically won't matter if you live in Southern Spain.

As to the bypass pressure, that's going to have a lot to do with the oil temperature. The thermostat regulates the coolant to a much narrower range than the oil, so on a hot day at the track you might see bypass pressure at 6,000 RPM while a cold day in the Alps you might hit bypass pressure at 4,500.

Its hard to pick an oil which will be good for both short trips and track days, but my advice is don't over think it, just go with the 5w40 and do an oil analysis for peace of mind.
gpshumway is offline   Reply With Quote
Old 08-23-2011, 08:20 PM   #3
Scooby Specialist
Member#: 41070
Join Date: Aug 2003
Location: Arizona
2005 Black Tiger
(Black Turbo Baja)


The less your oil pump bypasses, the more oil (flow) gets to the engine. I would suggest a quality 5W40.
bulwnkl is offline   Reply With Quote
Old 08-24-2011, 03:13 AM   #4
Scooby Specialist
Member#: 226211
Join Date: Oct 2009
Location: Europe
2002 impreza WRX JDM


yes, I mean the problem of pressure vs oil flow, we all know beside of 40 grade same technologycal and quality 10w40 always will be thicker than 5w40 in this case

Last edited by bako; 08-24-2011 at 12:29 PM.
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