Tranny oil ???

[TurboDuane]

When I had my tranny replaced, the shop told me they put in 30w Motor oil in it. They said it was the same viscosity as what Mazda uses/requires for their transmissions. Is this correct?

I'm now leaking fluid from the rear of the tranny, so I'm thinking I should replace the rear seal in the tranny, ...while I'm at it should I replace the oil? I've got a couple of quarts of Red Line in the garage, should I use it?

TD

[POS Racing]

I've been running 75w90 Mobil One in my fleet of Miatas. Are you sure they said 30w???

Factory spec. for a Mazda MX-5 Miata is:

specified oil: { API GL-4 or GL5 \ SAE 75w90 }
volume: { 2.0 L \ 2.1 US qt. \ 1.8 Imp. qt. }

[brock]

Someone told me the same thing at one point, but there are properties other than the viscosity that set them apart. Maybe someone else can chime in with the differences.

[wrxmr2eater]

The 75-90 gear oils are specified due to metal properties in a transmission. One of the biggest is the anti rust and corrosion abilities of gear oil over motor oil. It is in for a longer period of time so these become a much bigger issue in the transmission. Also the metals used in many tranmissions are not the same as in the motor internals and the lubrication is not as effective.

[TurboDuane]

I've been running 75w90 Mobil One in my fleet of Miatas. Are you sure they said 30w???

Yep thats what he said, I even asked a second time for clarification; as I was supprised by his answer when I asked him what they put in it.

I know most here are running redline or Moble One, but has anyone had any experiance the purple stuff? I think I'm going to invest in a seal puller. I've tried using the "screw a screw into the seal and attempt to pull them free" method, but I've always screwed up the metal around the seal. I don't want to do that again.

[POS Racing]

Before you go tearing the car apart, I wonder if the 75W90 would cure the leak?

I’d change the fluid first and see what happens.

[Darron65]

At the risk of being , I'll just chime in with my past life experience as a lubrication specialist for a large fleet.

I wouldn't worry too much about engine oil vs. gear oil in the transmission. Gear oils are almost always a GL-5 additive level and are really intended for hypoid gears in the axle. The gears in the transmission are not subject to the severe sliding/wiping action of a hypoid gearset and more is not better when it comes to extreme pressure additives. Many times the EP additives prevent good syncho performance and can actually attack some synchro materials (but I do not know if this happens with Miata transmissions). GL-4 is the compromise most manufacturers specify but it is hard to find. It represents 50% of the GL-5 treatment level.

So getting back on track, an SAE 30 engine oil has approx the same viscosity as a 75W90 gear oil but it has a narrower temperature range (not a multigrade). Plus the test temperature for the viscosity grade will not often occur in a transmission. I know of another Japanese car manufacturer that used to use 5W30 engine oil as factory fill in some of their fwd transaxles and maybe they still do.

The most important thing is getting the correct viscosity; second is getting the right additive chemistry.