Hello good people. I have a 1975 cutlass with a 350th transmission with Dexron Vl. My question is it ok to mix Dexron Vl with Dexron Vl / Merc LV? I read online its ok and the Dexron Vl / Merc VL is better to use. Can anyone tell me if this is true?

 

From what I've read about it on the specs, Merc LV has ever-so-slightly better cold flow, but the fluids are virtually identical. But Merc LV costs way more for whatever reason. Dexron 6 is pricey enough. Using either in your TH350 isn't required, but if you want, it's ok.

Dexron 6 is fully backwards compatible with the Dexron 3 requirements of your TH350. I wouldn't worry about the difference in the TH350. I'd just use the Dexron 6 and call it a day. But it's your car, your money. You do you.

 

Personally, despite backward compatibility of the newer fluids, I would stick with Dex III or Dex/Merc unless it’s a fresh build. I don’t see any advantage to running fluids designed for rather different, later transmissions.

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^^^This. I only use Dex III in my transmissions. Anything higher will have friction modifiers designed for newer cars and won't necessarily be correct for older transmissions. Dex III is backwards compatible down to the old Type A. Dex IV and up is not.

 

When the 4T60e in my 96 98 was on the way out I dropped the pan, replaced the filter and refilled it with Dexron VI. I had read on a GM H body forum that it had the powers to rejuvenate hard seals in old transmissions. No such luck.

I should have known it was a shot from the hip when I saw glitter in the pan when I swapped the filter. 😭

 

I habe mixed both Dex 3 and 6 with no instant failures. The friction modifiers is one thing but my main issue is a thinner fluid. I find those old transmissions leak fluid somewhere, somehow. A thinner fluid won't help. I actually used a combination of about 1/2 trans hydraulic fluid for tractors we sell, it is a 10W30 viscosity with some of our regular and some of are 100% synthetic Dexron 3 with some Lucas conditioner and stop leak. I just replaced the rear seal, no other leaks on this 4L80E. Something has to go right with this money pit.

 

Is dex III readily available at most auto parts stores?

 

You could at least order Dexron III from most auto parts stores. They would have it in their warehouse even if they didn't stock it in the store. It seems most auto parts stores only carry the universal fluids now.

 

 

In my simplistic mind, Dex VI is synthetic Dex III. All the pros/cons that come with being synthetic. If you prefer synthetic - great! If you prefer dino - great!
The "LV" stuff is totally different. Low Viscosity variants have started popping up for the transmissions starting in the 2010's (I think). Do NOT want to put a Low Viscosity oil in a pre-2000 transmission. Similarly, there are fluids designed for CVTs that you do not want to run in a traditional all clutch transmission.