Chimney Rock, NC -- October 2002
Miata Oil Filters, Dissected
last updated November 16, 2003

Fram Tough Guard, new Purolator Pure One, and a surprise bargain!

This is a cut-away comparison of a Bosch--supposed to be premium, $5.49 at Autozone--oil filter and the OEM Mazda filter ($4.50 or less online, $6.00 at your local Mazda dealer). Also shown is a Purolator PureOne filter for the Mazda Millenia (p/n PL14620) that is physically larger than the Miata filter but retains the same baseplate, mounting thread size, and relief valve pressure (14 psi)--so it will fit the Miata (at least the 1.6L engine--there may be space problems with the 1.8L?).


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Pictures 1. and 2. show the Bosch (left) & Mazda (right) oil filters:

Notice the superior design (recessed for better flow) of the Mazda inlet/anti-drainback valve section, this allows better flow than the flat Bosch design. The Mazda filter also uses a silicone rubber anti-drainback valve which is (according to some) superior to the nitrile rubber valve of the Bosch--the silicon rubber should remain flexible, and therefore more effective, at lower temperatures. Also note the much larger perforations in the core (outlet) of the Mazda filter.

The larger area of the Mazda filter element is obvious in picture 2., it's nearly twice the size. I have no way of scientifically analyzing the actual filtering performance of the filter media, however in terms of close examination "look and feel" they appear to be made of the same or very similar material. The better designed inlet/anti-drain back valve section, the larger filter element, and the larger core perforations create a much freer flowing filter--which means less pressure drop across the filter, and more pressure (and more oil) making it to the engine's oil galleries. This is why the oil pressure reading (the pressure sensor is located after the oil passes through the filter) will be higher using the OEM filter.

Pictures 3. and 4. show the Purolator PureOne filter for the Millenia (p/n PL14620). It has the recessed base design and silicone rubber anti-drainback valve of the Mazda filter, and although it shares the smaller core perforations of the Bosch filter (and most if not all other aftermarket filters I've seen), the potential flow restriction that this could create are compensated for by the significantly larger core/filter.

Fram Tough Guard (TG7317) -- A horribly constructed and overly restrictive piece of crap!


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Pictures 5., 6., 7., and 8. show a dissected Fram Tough Guard filter for the Millenia (p/n TG7317)--this filter is an absolute P.O.S.  The element has cardboard end caps, the pleats are not even close to evenly spaced, some of them are jammed up side-by-side, significantly reducing the effective filtering area.  In operation it's excessive pressure drop reduced the oil pressure reaching the engine on my Miata by 10-15 psi! 

Giving credit where it's due I will commend them on the use of a screen on the bypass valve, however this is probably something they had to do because the filter is so restrictive that the bypass valve is open half the time.  They also use a silicone rubber anti-drainback valve which is arguably better than nitrile rubber.  Picture 8. shows the other side of the  element, revealing more jammed together pleats.  

The base, can, bypass valve and anti-drainback valves aren't bad; so why is the element such a pile of rubbish?  Keep in mind, this is Frams's "premium" filter...imagine what their standard grade units must be like.

Purolator Pure One (PL14610) -- This has been my filter of choice for over a year...


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Pictures 9., 10., and 11. show the insides of a  Purolator Pure One for the Millenia (p/n PL14610).  This is Purolator's replacement for the PL16420 I dissected above.  As far as I can tell it's basically the same filter.  It seems to offer no more restriction than the OEM filter, it's construction and choice of materials are quite superior to those of the Fram.

Picture 12. shows the Advance Auto TotalGrip (p/n AA7317) next to the Purolator PL14610--you can see that despite the Advance Auto filter's Fran part number heritage it is NOT a Fram filter.  It is in fact identical to the Purolator Premium (L14610) standard grade filter.  More about this bargain below.

Advance Auto TotalGrip (AA7317) -- What I use now.


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Pictures 13., 14., and 15. show the Advance Auto TotalGrip (p/n AA7317) filter for the Millenia.  As I stated above, this is NOT a Fram filter and is in fact identical to Purolator's Premium (p/n L14610) filter--but at $2.48 it's quite a bargain.  My oil pressure readings indicate that it is no more restrictive that the OEM filter, the same as the Pure One. It uses a nitrile rubber anti-drain back valve and different media than the Pure One, however the bypass valve, base and can appear identical.  It is also of two or three orders of magnitude better construction than the Fram.  The filter also has a textured finish on the can (picture 16.) so that it can be installed and removed by hand--this is cool!

I change my oil and filter every 3,000 miles, so the nitrile rubber anti-drainback valve doesn't scare me, they claim that the filter media meets or exceeds manufacturer's recommendations, and it keeps the oil pressure where it should be--all for $2.48, you can't beat that!


This is the engine lubrication flow diagram for the 1.6L engine. Note that as indicated above the oil pressure sending unit measures the pressure after the oil has passed through the filter (I.e. it reports the pressure at the inlet to the engine's oil gallerys--as it should). Therefore filters with higher flow restrictions will reduce the pressure on the important side of things.

A Great Oil Filter Study

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