Thicker differential oil for Mitsubishi Montero

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I'm currently running GL-5 80W-90 gear oil in my front and rear differentials. My drain intervals are 40,000 km (25,000 miles), both take about 2.5 litres. Due to excessive heat, I need to add about 800 ml or so at the 20,000 km (12,500 mile) mark. This is common on every vehicle here, not just my application. I'm considering a switch to synthetic, because I reckon they would handle the heat better...but I'm also wondering if it would be safe to switch to something like SAE 140 or 75W-140. I reckon the monograde would be a better choice...but I would really appreciate some input. For the record, manual recommends 75/80W-90 or SAE 90. Thanks!
 

Falcon_LS

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Thanks, Pablo! I'll have a look to see if that viscosity is one of the available ones though. 80W-90, SAE 90 and SAE 140 are the most common ones I've seen at this end. Motorcraft 75W-140 synthetic is actually the one I was considering using. If I cannot find 75W-110, would this be a suitable alternative?
 
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a 75W-140 would be just fine. remember, multiweight has the viscosity properties of the lower number while providing the protection of the higher number.
 
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My vote is to simply use a full synthetic 75w140. With your driving conditions, I would still check the fluid halfway through the interval. Hopefully, the amount you top-off with is less then the 800ml you need now.
 
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