"API viscosity ratings for gear oils are not directly comparable with those for motor oil, and they are thinner than the figures suggest. For example, many modern gearboxes use a 75W90 gear oil, which is actually of equivalent viscosity to a 10W40 motor oil."
Several instances around this forum, people use a specifically designed 5 or 10W40 oil from Europe in the getrag for racing. Since mines a DD, I use the ford honey.
TYPICAL TECHNICAL PROPERTIES
European Car Formula 100% Synthetic 5W-40 Motor Oil (AFL)
Kinematic Viscosity @ 100°C, cSt (ASTM D-445) 13.7
Kinematic Viscosity @ 40°C, cSt (ASTM D-445) 82.9
Viscosity Index (ASTM D-2270) 171
Viscosity CCS, cP @ °C (ASTM D-5293) 5314 (-30)
Flash Point °C (°F) (ASTM D-92) 228 (442)
Fire Point °C (°F) (ASTM D-92) 244 (471)
Pour Point °C (°F) (ASTM D-97) -43 (-45)
Noack Volatility, % weight loss (g/100g) (ASTM D-5800) 7.4
Four Ball Wear Test (ASTM D-4172B @ 40 kg, 75°C, 1200 rpm, 1 hr), Scar, mm 0.44
Total Base Number 8.0
HTHS (cP) (ASTM D-4683) 3.8