. . . and not only in the way it’s made. The way it works is different too.
Some time ago I opened, examined, and tested several engine oil filters to see what differences there were. My main focus was on the bypass valve because in a number of Subaru forums there were claims that Subaru used a higher bypass pressure specification. My tests confirmed both that this was true and that some commonly used aftermarket brands did not meet the same bypass specification. This was reported in the thread “Wisdom of Using OEM Oil Filters ONLY”, started by 96OBWin NY, in April 2009 (Wisdom of Using OEM Oil Filters ONLY
In addition, there was, and continues to be questioning about the Subaru transmission spin-on oil filter – is it just an engine oil filter that fits, or is it really different? I finally got hold of a transmission oil filter and can say without doubt it is physically different, and based on this difference, it also functions differently. Here goes . . .
The filter I examined is an original black Tokyo Riko filter that is marked for use on the Automatic Transmission. This one had been removed from a 1999 Outback 4EAT that had delayed shifting problems.
Externally, and for the most part, internally, it looks the same as the black Tokyo Riko filter that is used for the engine (or at least was available until last year) but has since been replaced by the blue Honeywell-made filter (at least in North America).
Transmission filter -- looks pretty standard:
Even the inside, initially is much the same. Here's both with mounting caps off (engine filter on the left)
However, there is a very noticeable difference when looking inside. The engine filter has a visible metal “tower” in the middle which encloses the bypass valve spring. The transmission filter, instead, has a mesh screen (engine filter on left, tranny filter on right):
Furthermore, with the filter medium removed from the can, on the bottom of the engine filter one can see the plate that forms the valve over the bypass hole. On the transmission filter there is no blocking plate. The hole is open right through to the mesh. This difference is very significant.
Here I have a light on the other end of the filter showing that the hole is open to the mesh and the filter outlet pipe:
I could find no other apparent distinction between the two. The filter media is held in the same metal end caps; both have what look like identical anti-drainback rubber seals; both have the same six inlet holes; both have the same spring in the bottom of the can holding the media and anti-drainback valve against the mounting plate; and, both have the same mounting thread. (I have no way of telling if the filter media itself is in any way different – visually it isn’t.)
What makes the tranny filter so different from the engine oil filter (regardless of brand) is the fact that the hole leading to the mesh is not closed by a valve, as in the engine filter. This means that there is no restriction to the flow of ATF, other than the size of the hole, through the mesh. There is a constant flow of ATF through the hole and the mesh, not through the filter media! It might even be that it’s only when the flow increases to beyond that allowed by the hole and mesh, or the mesh itself becomes blocked, that the bulk of ATF “bypasses” through the media. This is the reverse of the bypass function of the valve in the engine oil filter.
This further suggests that the use of a non-Subaru filter based on an engine filter, that has a bypass valve or that is full flow through the media alone without a bypass could excessively limit ATF flow to the transmission under normal operation. It also might explain why the ATF filter is changed only when needed, such as a transmission internal breakdown when actual pieces might block the mesh. Barring this, the filter should pass ATF through with little or no restriction for a long time.
Of course, this doesn’t preclude the possibility of changing the ATF filter more frequently, but the transmission filter is less likely to present an impediment to ATF flow even over a lot of miles, unlike an engine oil filter which will tend to clog progressively due to the combustion by-products that can contaminate the engine oil during use, and build up on the filter media, restricting flow and causing more (and undesirable) “bypass” operation.
Well, that's the way I see it.