Mazda MX-5: Diff Fluid Change With Castrol Syntrax 75W-90 Synthetic

Another unseasonably warm day this February with tropical like heat wave weather so I did some more maintenance on my Miata in preparation for the Road Trip to Florida next week.

Was the gear oil due for a flush? Nope… not necessarily. I was about 9k miles short of the scheduled change. However, I thought I heard a little whine coming from the rear end on a recent trip down the Jersey Shore for the Millenials Miata meet, so I decided to flush out the Mobil 1 I put into the diff at the end of 2015 and replace it with Castrol Syntrax 75W-90 synthetic.

As usual this was a two step process. Once I decided on the Castrol (and part of my decision was based on the engine oil I just started using – Castrol EDGE… not that engine oil has anything to do with the diff, I figured I’ll keep things consistent), I researched where I could buy it. First I found O’Reily Auto Parts sells it for about $14 bux but we don’t have any of those stores around here. Pep Boys and Advance Auto did not advertise it on their web site, Walmart could place an order for me for about $13 but surprisingly Auto Zone had it in stock for $9.99! I still wanted it cheaper, so I went on CardBear.com researched AutoZone gift cards and bought one from Raise.com saving about $3 bux! The gift card showed up in just a few days since I ordered it and as soon as it arrived I rushed out the door to my local Auto Zone to pick it up.

It was definitely convertible weather today!

It took no time to jack the car up and get it leveled to do this job, but once it was in the air I could not (for the life of me) find the necessary 23mm and 24mm sockets to do the actual job! Fucking hell…. I searched everywhere, wasted more than an hour looking to no avail. So I went to the local Lowe’s to pick up two sockets so I have them for the future. I stopped by my local Sears to see if they had Craftsman in stock since I have some Shop Your Way rewards points to burn but the 23mm socket is a high commodity only available for online orders… the price for the 24mm socket was $12 or $13, I picked up both sockets at Lowe’s for under $10! I think they were $4.88 each plus tax which seems very reasonable. (Though I’m sure I could have bought them for way less if I had the luxury of time).

Anyway, the job itself took no time. Very easy to undo the top bolt, then the bottom to drain the diff. Cleaned everything up. Put the bottom bolt back on. Jerryrigged a device to pump the new fluid into the diff, and then tightened both the bottom and top bolts. Surprisingly I was able to pump almost the entire bottle in this time, I think I pumped less with Mobil 1 a year ago, but I could be wrong.

So there it goes. Easy job. I’m hoping this one lasts more than 20k miles before the rear end starts making whining noises again. I should mention I was very surprised how dark the Mobil 1 gear oil got… many of you will remember I said the same thing about my Mobil 1 engine oil the first two oil changes I did. I just don’t know what makes that oil turn so dark so soon.

Some people claim that the oil is so good it takes all the dirt out of the system that’s why it turns so dark. And that the other oils I use are not as good and therefore not as dark when I flush them. But I call BS on that! There’s no reason why the rear diff fluid should be this dark at just 20k miles. I think it should have lasted longer. I guess time will tell when I flush the Castrol Syntrax for a direct comparison.

I took a short 2 mile drive down to the local Advance Auto to responsibly dispose of the used oil, and the diff feels just fine. Nothing to talk about… it’s smooth, there’s no noise. It’s good!

Will follow up again when it’s time to flush it in 30k miles.