Subaru Impreza Oil Filter Comparison

Introduction

Since Subaru Impreza oil filters are so easy to get at and change, I've started doing my own oil changes. There's been a lot of discussion on www.impreza.net (and elsewhere on the net) on the relative merits of OEM vs. various aftermarket oil filters, with the general conclusion that OEMs are usually decent, el cheapo Frams are usually too cheap, Mobil 1s are good, and others vary. However, it's also clear that the construction of different models of filters can be different, and can change over time.

Therefore, I decided to cut open the OEM Subaru oil filter and a few others, take pictures, and post the results. Disclaimer: these results and opinions are solely mine; they may or may not represent the opinions of Subaru, Purolator, Fram or others.

Armand MacMurray
armand1[impreza.net]
January, 2003

What I Looked For

Since the Subaru oil filter is mounted "upside down" at the bottom of the engine, an anti-drainback valve is not necessary, as gravity will keep the oil in the filter when the engine is off. However, a good bypass valve that doesn't pass unfiltered oil until the specified high pressure is reached is as important on Subarus as on other engines. A well-constructed filter cartridge, with a leak-free connection to the central outlet threads is important, as are good filtration media and plenty of surface area in the intake holes around the perimeter of the filter end and the holes in the metal scaffold around the central core. The latter two sets of holes, along with the surface area of filtration medium, minimize the pressure reduction caused by resistance to flow inside the filter

Filters Examined

Subaru OEM (made in USA by Purolator)

This is the Subaru OEM oil filter, purchased from my local Subaru dealer for about $4.50 each in bulk. It's made in the USA by Purolator.

It seems well-constructed from the outside, with a rounded black rubber sealing gasket surrounding the 8 intake holes. Inside, it consists of the following components, from intake end to closed end:

  1. Thin outer inlet plate
  2. Heavy-duty inner inlet plate
  3. Rubber gasket
  4. Metal filter cartridge
  5. Metal arch spring

A and B are securely glued and crimped together into one unit; A is also crimp-sealed around its edges to the outer "can" that holds all the other components. D has metal end-caps and a short central neck on the inlet side; the gasket C is compressed between the threaded central neck from B and the neck from D, preventing any leakage between the dirty intake side and the central clean oil output (the ends of the two necks also nest, providing additional mechanical strength to that seal).

The cartridge D contains 56 pleats of filter material between the metal endcaps, and a spring loaded bypass valve inside the end of the central core. The cartridge is held in place against the inlet plate and gasket by a metal arch spring that presses against it and the inside of the canister. The bypass valve has a stiff spring.

PureOne PL14460

This is the premium Purolator PureOne oil filter, purchased from my local Fred Meyer store for $5 (also found in the PNW at Pep Boys, GI Joe's, Sears and others).

It seems well-constructed from the outside, with a flat orange rubber sealing gasket (claimed to be teflon-treated) surrounding the 8 intake holes. Overall, it is the same shape and size as the OEM filter (but painted to match World Rally Blue). Inside, it consists of the following components, from intake end to closed end:

  1. Thin outer inlet plate
  2. Heavy-duty inner inlet plate
  3. Metal filter cartridge
  4. Metal arch spring

A and B are securely glued and crimped together into one unit; A is also crimp-sealed around its edges to the outer "can" that holds all the other components. C has metal end-caps and a short central neck on the inlet side; this neck is glued to the threaded central neck from B, preventing any leakage between the dirty intake side and the central clean oil output (the ends of the two necks also nest, providing additional mechanical strength to that seal). There is no gasket, as in the OEM filter.

The cartridge C contains 63 pleats of filter material between the metal endcaps, and a spring loaded bypass valve inside the end of the central core. The cartridge is held in place against the inlet plate and gasket by a metal arch spring that presses against it and the inside of the canister. The bypass valve has the same stiff spring as the OEM filter.

Fram PH3593A

Fram has a number of different filter lines. This "Extra Guard" seems to be their entry-level filter. It ran me about $3 at my local discount auto parts store.

It seems well-constructed from the outside, with a flat black rubber sealing gasket surrounding the 12 intake holes. Overall, it is about 1/2" longer than the OEM filter, and does have a handy, rough "grip" coating on the end. Inside, it consists of the following components, from intake end to closed end:

  1. Thin outer inlet plate
  2. Heavy-duty inner inlet plate
  3. Rubber combination anti-drainback/outflow-sealing gasket
  4. cardboard filter cartridge
  5. Combination bypass valve/arch spring

A and B are securely glued and crimped together into one unit; A is also crimp-sealed around its edges to the outer "can" that holds all the other components. D has cardboard end-caps and no central neck on the inlet side; in order to attempt to seal the inner (clean oil) core from the outer (dirty oil) inlet, the inner cardboard rim of D presses C against a small raised area of B between its inlet holes and the outlet hole. The bypass valve/spring combo E seats in the central hole at the other end of the catridge.

The cartridge D contains 40 pleats of filter material between the metal endcaps. The number of small holes in the metal cylinder lining the inside of the cartridge (through which filtered oil flows into the core) is also noticeably fewer than in the Purolator filter cartridges (this might in theory increase the filter's resistance to oil flow). The cartridge is held in place against the inlet plate and gasket by the combo bypass valve/arch spring that presses against it and the inside of the canister. The bypass valve part of this has a much weaker spring than either the OEM or PureOne filters.

Conclusions

The construction of the Fram filter, with its reduced filter media area, its uninspiring cardboard filter cartridge, its lack of nesting metal necks for strong sealing of the clean oil outflow route, its weak bypass valve spring and other perceived weaknesses, lead me to rank it last among the three filter examined here. I personally wouldn't install one in my car even if they were giving them away.

The Subaru OEM and PureOne filters are very similar, and both made by Purolator. In my opinion, neither is clearly superior to the other. Both are constructed well, and have robustly-sprung bypass valves. The main factor in favor of the OEM filter is its rubber sealing gasket compressed between the two oil outflow necks, ensuring a good seal. However, as the PureOne has some adhesive in the same place (in place of the gasket), I would think that it is also sealed well in almost all cases

The PureOne filter has about 12% more filter surface area, and may have a filtration material superior to that in the OEM filter. The material in the PureOne looked very similar to that in the OEM filter, so I could not determine visually if they are different.

Since both the OEM and PureOne filters seem essentially the same, I would just use whichever is cheapest/most convenient to buy. In the Pacific Northwest, many stores such as Fred Meyer, GI Joe's, Sears, Pep Boys, etc. carry the PureOne filter at about $5 each, so that's the one I usually use in my '02 Impreza OBS.


Contents Copyright (C) 2003 by Armand J. MacMurray

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