View Full Version : Mini Truck tranny & t-case fluid?

02-17-2008, 10:40 AM
What king of fluid are you mini truck guys using in your manual tranny & t-case? I want to find something that works well in Colorado's hot and cold extremes. I'm thinking synthetic 75-90, but do you prefer one brand over another? Is it worth it to get the Redline?

Thanks for your help!

02-17-2008, 11:11 AM
Redline MT-90. Nice if your rig features worn out sycros.

Just don't run mobile 1 in the transmission. It is too slippery.

02-17-2008, 11:58 AM
I've always run Castrol Hypoy-C 75W-90 in the tranny and transfer and 80W-90 (when I can find it, otherwise the same 75W-90) in the axles. Just bought synthetic for it all. Went with Redline MT-90 75W-90 for the tranny and xfer and standard 75W-90 for the axles, mostly on the advice of leiniesred, Red_Chili and the mini truck collective.

02-17-2008, 01:54 PM
Ok, so the hype is backed by truth. Thanks! On cold mornings my synchros are still sleeping for a few miles; I have to be patient. I am willing to spend a little more for good results. Is synthetic really necessary in the diffs? Also, are there any local shops that carry Redline at a reasonable price, or do I need to order it online?

02-17-2008, 03:57 PM
Here's an older thread, there are a couple of places locally. Same price as Summit (usually about $9/quart).


02-17-2008, 04:07 PM
^^ The link at the bottom of that thread to 4x4wire is interesting. Trannys are still the most mysterious part of a truck to me. I've never torn one down, nor do I want to.

A few close shops there too. I'm going to try my local race shop and see if they have it. To do just the tranny/t-case, I need 5 qts?

I'm still curious if it's worth it to throw the $$ into synthetic diff oil?

02-18-2008, 10:01 AM
I don't know if it is worth it or not. let's face it, must trucks on the road run regular oil for hundreds of thousands of miles without trouble.

My emperical evidence that synthetics work:
Switching to all synthetic oils/greases got my truck 1 more MPG.
Switching to synthetics lowered my rear diff temp 10 degrees F. on the current set of 5.29s I built.

I'm pretty sure my rear diff. is mobil 1 90w-140 right now. I noticed FORD is spec'ing 70W-140 synthetic for their differentials these days.

TIP: Gear oil can be a real pain to add. Some pull the shifter and dump it in. My big tip is to warm up the oil before you start pumping it in. It is just easier. I use a funnel in the wheel well and a long hose to the fill hole. it takes a long time to fill it this way, so I do other stuff while I keep filling up the funnel. Warm oil flows in much faster than garage cold oil.

02-18-2008, 10:14 PM
Hmmm... got the front Detroit and 5.29 installed today, and I'll be getting the rear E-Locker and 5.29 next weekend... I hope. So lets pray for no need to use 4wd, as 5.29 in front and 4.10 in rear would not be good! Also, I rebuilt the3 front hubs (with the help of wiser friends); boy is that a messy job!

Anywho, I'll be breaking in these gears, so I'll use the plain 80-90 I had lying around for the break in, and in 500 miles I'll try the synthetic. She's an old girl, so I want to take good care of her!

02-19-2008, 10:46 AM
I sometimes don't bother with synthetic in the diffs - it seems I end up changing it all the time, which if you run water crossings is a good idea in itself. However, I do run BG MGC if I am running dino oil.

The stuff works. Tested on a Honda thumper that was notoriously hot running. This reduced the temps for my right ankle to a noticeable degree, no other changes. That is significant!

02-19-2008, 11:00 AM
JKimmel taught me to use the BG MGC additive to keep the temps down during break-in. Might be a waste, but all of the diff's he and I have built are still working (so far).

BG Part number: 325
Available at NAPA.

02-19-2008, 01:33 PM
I sometimes don't bother with synthetic in the diffs - it seems I end up changing it all the time, which if you run water crossings is a good idea in itself.
This has always been my thought. I'm doing the Redline in the rear axle for the trip out to Rubithon 'cause it's a lot of desert highway miles.

02-19-2008, 03:05 PM
So you'd use the BG only for breakin? I may look at that.