Transmission and differential fluid questions
 

I've done some research in the passed few days here while I'm gearing up to replace my clutch, and the topic of which fluids to use in my differential and transmission has come up.

From what I'm seeing, differentials with LSD (which is what I have) seem to work best with 75W90 gear oil, with a slight ammount of LSD fluid additive in them. The more fluid you add, the more the differential slips, but if you don't use enough fluid, the differential will chatter and shudder. Is there any certain ammount you're supposed to use? Also, I remember seeing Mobil 1, Redline, Neo, and Royal Purple being popular choices for fluids, and it seems each of these respected companies carries the LSD additive too. I'm leaning more towards Mobil 1, because I've basically raised myself on their products and have never been let down with their quality or price.

Now for transmissions, MT-90 transmission fluid has been a popular choice, including such companies as Redline, Royal Purple, and Neo. I've read that Redline is a bit thinner fluid than the RP or Neo, which is good in the sense that if you live in colder weather, that it's going to help protect your transmission better and improve shift rate because it's not so thick. Neo and RP are supposed to reduce grinding because they are a thicker fluid, but also supposed to be able to improve shift rate also, but not improve as well as Redline fluids would.

Actually for diffs if you are using a conventional oil, you would want to use 85W90 or straight 90W.

You only add a LSD additive, if you have a LSD. If you have a open diff, you are only throwing away money putting in a LSD additive into the differential.

MT90 would be fine in both the differential and transmission for most street driven applications. The only exception to that would be if you had a LSD, then you would want to use either Redline 75W90 or Redline 75W90NS (with a limited slip additive), or some other 90W style gear oil.

I highly do not recommend the use of Mobil1 gear oils in the transmission due to what happens when it shears or overheats (anything continuas over 220F/225F causes the gear lube to start breaking done releasing acids which will damage the transmissions syncros- granted that is in extreme cases, that the gear lube every exceeds 200F, but it is something to consider).

I have an LSD in my GXL, and plan on using the 75W90 with the LSD additive in it. I planned on using the Mobil 1 75W90 full synthetic gear oil with a LSD additive in it, but just wasn't sure on how much to use. I debating on using Redline oil though instead...

I don't plan on using Mobil 1 gear oil in the transmission, but instead using Redline MT-90, or this shock-proof stuff I've been hearing about.

I just want to use the best lubricants I can get that are going to give me the best performance. Seems like Redline is going to be better option for me since I live in a little bit colder climate in Iowa, and need something a little lighter than Neo or Royal Purple. It's also said the Redline shifts faster than Neo and Royal Purple, but doesn't give as good of a protection against grinding. Tough decisions....

learning how to drive your stick is the only thing thats going to allow you to shift faster, if your trannies in good condition then the thing you need to worry about the most is driving it right like no grinding between shifts. Pretty much all oil's are soo close you wont notice a diff between them i have noticed a diff using synthetics though, in my motor oil,but in my tranny i havent noticed any diff, only that its more exspensive when i change the fluid. In my diff which is lsd i have 90w with lsd additive and its worked great for about a year now.

I use Redline 75W90NS in my diff and I do not notice any significant chatter or noise (Redline warns that NS has no slip modifiers and might make noise - but it is recommended for the track)

15-20 year old LSD's are usually pretty worn and not too picky on LSD-additive level. Add it only if you need too.
Most gear oils have LSD additives already added. So, try the Mobil1 without using any LSD additive. Good thing about M1 is that it is available at most autoparts stores. The M1 has never failed me in the multiple rear/front ends, manual trannies, and transfer cases that I've used it in. Royalpurple is also available in multiple stores now. The RP Maxgear is another excellent choice for the rearend.

You can't go wrong with the Amsoil MTG or Redline MT90 in the manual tranny.

I wouldn NEVER use MT90 in the rearend. It is a GL4 oil and not the required GL5. If you use Redline, Use the Redline 75w90 for the rearend.

A 30k/3yr change interval, regardless of fluid brand chosen, should be considered. .

Redline shock proof is probably too slippery for use on such a low power application that is in a tranny with syncros.

So, where do you get redline stuff? No one around here carries it. Am I just unlucky, or do you all order it online?

myoilshop.com and a bunch of online stores like jegs, summitracing,.....

Ok, I was just wondering if there was any particular one people had success with.

You usually won't find redline at an autoparts store. Most good speed shops carry it.

Yeah, all we have around here is shitty ricer shops. O'reilly's does carry royal purple, but its damn near 12 bucks a quart for the tranny and diff fluids.

Whatever you do... don't use Valvoline.

^+1. Stay away from cheap synthetics.

Regular Redline (10W30) is $10.50/quart (plus tax) at Partsource here in Toronto.

I went ahead and ordered from myoilshop.com. I had already decided to use either redline or royal purple from what I had read. They actualy had a few quarts of redline motor oil at the advance auto here, it was I think 9.50 a quart for 20W50.

Redline, Royal Purple, Neo and certain others will let you shift faster. They claim it, explain why, people have tried it and so have I. I have Neo and it shifts way faster and easier - seems to pop into gear instantly to me but I'm no racer (I can't distinguish 0.1 seconds vs. 0.2 seconds, etc.).

www.racingbeat.com has Royal Purple for $5.50 a quart (all motor oil weights). RP Max Gear (for the diff) is $7.50. I believe you need Synchromax for the tranny, which I didn't see on their website.

Im sorry but i cant believe that..it doesnt allow you or let you shift faster,you can only shift as fast as you can put the car into gear period. Oil's only job is too provide protection against heat and friction,the tranny is not dependant on it's use to function,hence why you can drive fine for a little while without gear oil. Ive done it for about 10miles.When i realized what happened i put gear oil in and it was fine luckly.

Redline has a good little writeup explaining why. Mostly it's because lower viscosity gear oil slips between the synchros faster so they engage quicker. I can in fact put the car into gear faster. There is less resistance so the shift lever flies right in to gear almost as if wasn't connected to a tranny. This isn't some imaginary wishful thinking, the difference is noticeable and huge. I've tried it, so have others.

Alright, looks like I'm going to try the 75W90 Mobil 1 gear oil in the rear end and see how it works. If I chatters, I'll put some of that LSD additive in it. Also, I plan on using the regular Redline MT-90 in the transmission and see how that works. Any other suggestions?

This might sound dum but can I use Redline 75W90 in my tranny or do I have to use the MT-90?? Just wondering, I have like 8 bottles of the 75W90. Any help, Thanks in advance.

Yes, you SHOULD use MT90 or 75w90NS in the manual. The Redline 75w90 has the LSD additive in it. You won't have optimal shift feel, and could possible cause gear crunching in the typically tired Rx7 transmission.
The 75w90 will work, but I would use it as the last choice!

Well, I'm about to put in the tranny and diff fluid tomorrow. I'm going to use the beer bong (funnel and tube) method. Hopefully it will work ok.