May have overfilled Manual Transmission?

I swapped out my manual transmission fluid for Redline MT-90. I know that the user manual for the speed 3 says that the transmission takes about 2.6 quarts of fluid, so I bought 3 quarts of the Redline. Everything went fine, but i had read online and seen online countless times that you’re supposed to fill up the transmission through the fill hole until it starts to overflow. First two quarts, no problem. I get to almost all of the bottle gone on the third quart, and it never overflowed through the fill hole. I was really confused, but I figured i probably shouldn’t put in anymore despite it not overflowing, so I put the fill bolt back on. I’m now conflicted because I know that overfilling a manual transmission can cause damage, but I’ve ALSO read that you need to keep filling it until it overflows. So what do I do? I took it out for a drive, everything seems fine other than the LSD whine that comes with switching to the Redline, which is expected and not harmful on my vehicle.

Yes, I put the Redline in the correct hole. I didn’t put tranny fluid in my engine or otherwise. And yes, the drain plug was on before I put in the new fluid. :slightly_smiling_face: Thank you!

That’s with the car sitting flat and level.

If you have one end or side of the car jacked up to get under it, the car is not sitting level.


I thought of this, and took the car off jacks. still no overflow when it was leveled.

What is LSD whine and why did you deliberately cause it. Is this a car you are going to race?

Limited Slip Differential whine. Some cars have gear-type LSD’s (Gleason-Torsen or Quaife) from the factory. Of course you can also install aftermarket units made for the car. They are especially effective in front-wheel-drive cars since they don’t fight the steering like a clutch-type LSD.

Remove the fill plug and stick your finger in and point it down. If you touch oil you’re OK.


I wouldn’t worry too much about this. Overfilling a manual transmission isn’t likely to cause damage.

The last time I changed manual transmission oil, I had the front end on ramps, so I jacked up the back end to make it approximately level as I filled it, but it isn’t a precision kind of thing like it is with an automatic transmission. As long as the gears are bathed in oil, you’re good.


I don’t race my cars, I just like having smooth shifts - so I switched to Redline.

Okay, will give this a try. If I don’t feel oil, I should add more accordingly I’m assuming?

Haha, I always worry about my cars :rofl: but thank you!

You haven’t told us what car you have, year, make, model. But some manual transmission have two bolts that you remove. Actually three if you count the drain bolt. The two fill bolts are apart lengthwise but one is slightly lower than the other. You remove both and fill from the top until the gear oil or ATF comes out the lower hole.

2008 Mazdaspeed 3 with 132k miles.

That car was configured with two manual tranmssion options

g35mr – capacity is 3.03 quarts
a26mr - capacity is 2.6 quarts

So maybe you have the first one, which would take more to refill it. In any event provided you are using the correct fill hole – which I expect you are – and you don’t feel the oil through the fill hole after driving the car a few miles to get the oil to redistribute the oil, then just buy another bottle of trans oil and top it off. Usually the amount of anglular tilt involved when jacking the car up doesn’t cause enough error at the oil level fill plug to worry about. If you can feel the oil inside that hole, insert the plug and you are good to go.

BTW, me, I’d only use the exact transmission gear oil as spec’d by Mazda.

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Oh trust me, I’m one to always go OEM too but I’ve done a lot of research and the OEM fluid has friction modifier, which quiets the LSD down but because it makes the fluid so slippery, it really messes with the synchro’s in the transmission and causes some rough shifting and wear. The Owner’s manual for the car states that the transaxle requires 75w-90 gear oil, which MT-90 is. So, I am in spec.

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That’s good. My owner’s manual for my Civic says to use Honda brand manual transmission oil, or 10W-30 oil for shorter periods. I’ve been using synthetic 10W-30 for at least a decade with no issues, changing it as often as Honda recommends changing its MT oil.