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!