Manual transmission and filters

none of the various manual cars I have owned (Ford, Hyundai, Porsche come to mind) have had a transmission oil filter.

is this a general phenomenon - or some do some don’t? why not have a filter standard like on the engine?

All of the gearlube is self contained in the transmission case. More modern units use automatic fluid, but the older boxes, such as the Muncie used 90 weight gearlube. That’s pretty thick stuff to filter out. Unlike an engine which breathes in air (Along with dust, etc.) A manual transmission is self contained and sealed off, and pretty well protected from outside contaniments. Doesn’t really need one.

I’ve never seen a manual with a filter, there’s no pump, so no way to run the fluid through a filter. Any metal particles typically sink to the bottom.

That's pretty thick stuff

makes sense

Any metal particles typically sink to the bottom

until the shaft spins, then it gets all over the place… so when changing this oil, is it even more important to perhaps do a couple flushes, and to aspirate the sludge somehow to get it all out?

Many have magnetic drain and fill plugs that capture any tiny particles…I sometimes take it a step further and insert a super-powerful rare-earth magnet to positively trap any iron / steel particles…When a rear wheel bearing failed and released much such debris into the rear end of a Crown Vic, I placed two such magnets inside the rear cover and they did an excellent job of trapping all the filings from the failed bearing…

Oil filters are needed on engines to remove contaminants from combustion blowby and oil oxidation at high temperature. Those are not a problem on manual transmissions.

I placed two such magnets inside the rear cover

yes my engine oil drain plug has a magnet on it, but not sure about the tranny - good idea though.

Oil filters are needed on engines to remove contaminants from combustion blowby and oil oxidation

so are you saying if the engine is “shedding” metal, an oil filter isn’t going to help?

I’ve never heard of a manual xmission car having an oil filter for the xmission fluid. But I did a little Googling, and it appears Mitsubishi may make a car that has one.

The manual transmission of my Civic uses 10W-30 motor oil, but I can think of a few reasons most manual transmissions don’t have filters. There is no oil pump. The oil gets circulated by the movement of the gears, so there is nothing to push the oil through a filter. Basically, the gears sit in a bath of oil. If you use the clutch correctly, you shouldn’t grind the gears, so there shouldn’t be anything to filter out of the oil. Some cars have a magnetic oil plug that catches metal fragments, but that isn’t the case on my Civic, so I change the oil every 30,000 miles. It only holds 1.9 quarts of oil, and it’s very easy to change since I’ve replaced the drain plug with a valve.

Magnetic drain plugs are available to fit almost any application…

That's pretty thick stuff

Not all manufacturers used thick gear oil for the manual transmissions. For years GM’s manual transmission used Dexron automatic transmission fluid. That stuff is very very thin.

75w-90 gear oil is about the same thickness as 20w-50 motor oil.

If a Honda Civic uses 10W-30 motor oil in its manual transmission, why don’t they just follow common motorcycle practice and use the same oil for the engine and gearbox?
I think that Ford’s Model T had a transmission that shared oil with the engine.

The crankcase oil is pretty quickly contaminated, that would require changing all the oil pretty frequently, and it would require a different engine design for the automatic transmission (which most motorcycles don’t have).

The Honda CB750A, sold from 1977-1979, used engine oil in the transmission and torque converter. This bike had a two speed semi-auto transmission with torque converter.
Even manual transmission bikes have oil bath wet clutches, similar to the cluthes in automatic transmissions.
That wet clutch is the main reason that oil used in motorcycles needs to have the JASO MA rating on the label.

Yeah, I did mention that in the same post.

Thinner oil works better in roller bearings,besides most ATF has extreme pressure additives.A lot of the old school around here thinks oil has to be thick to lubricate(mollases are thick, but a poor lubricant)-Kevin


Honda doesn’t call it 10W-30 oil. They sell 10W-30 oil with additives in it, and they call it Honda Manual Transmission Fluid so they can sell it for more. I typically use synthetic 10W-30 oil in my transmission, but I use regular dino oil in the engine.

I’d imagine they don’t use the same fluid for both because it would raise the cost of each oil change (because you’d have to add about two quarts to each change), and the MTF doesn’t need to be changed as often. They probably also do it because the engine calls for a different oil (5W-30). Newer Civics use 0W-20 oil in the engine, but they probably use the same 10W-30 in manual transmissions.

My motorcycles typically hold less than three quarts of oil, so using the same oil for everything isn’t going to greatly increase the cost of changing the oil. The capacity of the wet clutch and transmission combined probably amounts to less than a quart.

The controversy over oil viscosity seems to be deteriorating to an “us against them” battle. And even Mobil Oil struggles to be vague in their answers to consumers

but I often see factory oil recommendations that specifically recommend not using 0-W oils when operating at sustained high speeds or in extremely hot temperatures. It is highly unlikely that a long range test of engine longevity relative to viscosity will be taken so we can continue to bicker.

Motor oil = Shelf Space
Engine Life = Disposable Consumer Product…