I'm not sure if many of you guys noticed that Ford has made the change from 75w140 to 75w85 gear oil for the rear diff. Ford doesn't seem to release nearly as much information about their lubrcants as some manufactures do, but I was able to quickly find the following info.
MOTORCRAFT SAE 75W-85 PREMIUM SYNTHETIC HYPOID GEAR LUBRICANT
Kinematic Viscosity @ 100°C, cSt --- 11.53
Kinematic Viscosity @ -40°C, cSt --- 48,000
Viscosity Index --- 170
Pour Point, °F (°C) --- -49 (-45)
Motorcraft SAE 75W-140 Synthetic Rear Axle Lubricant
Kinematic Viscosity @ 100°C, cSt --- 25.6
Kinematic Viscosity @ -40°C, cSt --- 135,000
Viscosity Index --- 168
Pour Point, °F (°C) --- -49 (-45)
As you can see, there's a huge difference in viscosity between the two. One thing that really annoys me is Ford reporting their viscosity at -40°C (which interestingly is also -40°F). Gear oil is essentially solid at -40°C so you don't really get much useful information from that data. They also don't include any pressure tolerance info, which is essential for a gear oil. Motorcraft tends to use Group III base stocks in their synthetic oils, so I wouldn't expect those Falex figures to be stellar. For comparison here's a some info one of Amsoil's non-premium gear oils in a viscosity close to Ford's new recommendation:
Synthetic 80W-90 Gear Lube
Falex Procedure B (ASTM D3233) (failure load, lbf.) --- 2000
So taking a look at the specs it is instantly obvious that the 75w140 is much more viscous and can handle far more pressure than the 75w85. So why did Ford move to what appears to be a less robust lubricant for its newest Pony Car? I reached out to some engineers at Ford and here are the reasons they gave for the change; "75w85 axle lubricant has less parasitic losses (better fuel economy) and runs cooler than previous lubricants. The new lubricant is less viscous in cold weather. It is used on all 2015/16 Mustangs including the Torsen and limited slip applications."
So my initial assumptions were correct, much like the switch from 5w30 motor oil to 5w20 over a decade ago this change was driven by the goal of increased fuel economy. In this case, I agree with the change. For 99% of Mustang owners I don't believe they'll ever notice the difference, the 75w85 will be fine. Modern lubricants are far more robust than those from just 20 years ago, and for the vast majority I believe the new spec oil will meet all their needs. However, let's take a look at some Premium Synthetic options from Amsoil:
Amsoil Synthetic 80W-90 Gear Lube
Kinematic Viscosity @ 100°C, cSt (ASTM D445) --- 16.8
Kinematic Viscosity @ 40°C, cSt (ASTM D445) --- 109
Viscosity Index (ASTM D2270) --- 167
Falex Procedure B (ASTM D3233) (failure load, lbf.) --- >3500
Amsoil Severe Gear® 75W-140
Kinematic Viscosity @ 100°C, cSt (ASTM D 445) --- 27.5
Kinematic Viscosity @ 40°C, cst (ASTM D 445) --- 193.2
Viscosity Index (ASTM D 2270) --- 180
Falex Procedure B (ASTM D 3233) (failure load, lbf.) --- >3500
IMO, if you are adding significant amounts of power (150+ RWHP) to your S550 Mustang or using it in extended HPDE events I would suggest switching to a gear oil like Amsoil's Severe Gear 75W-140. You may loose 1/10 of a mile per gallon in fuel economy and 1 RWHP, but the added protection you would be giving your differential would more than offset those losses.
-SID297:beer:
MOTORCRAFT SAE 75W-85 PREMIUM SYNTHETIC HYPOID GEAR LUBRICANT
Kinematic Viscosity @ 100°C, cSt --- 11.53
Kinematic Viscosity @ -40°C, cSt --- 48,000
Viscosity Index --- 170
Pour Point, °F (°C) --- -49 (-45)
Motorcraft SAE 75W-140 Synthetic Rear Axle Lubricant
Kinematic Viscosity @ 100°C, cSt --- 25.6
Kinematic Viscosity @ -40°C, cSt --- 135,000
Viscosity Index --- 168
Pour Point, °F (°C) --- -49 (-45)
As you can see, there's a huge difference in viscosity between the two. One thing that really annoys me is Ford reporting their viscosity at -40°C (which interestingly is also -40°F). Gear oil is essentially solid at -40°C so you don't really get much useful information from that data. They also don't include any pressure tolerance info, which is essential for a gear oil. Motorcraft tends to use Group III base stocks in their synthetic oils, so I wouldn't expect those Falex figures to be stellar. For comparison here's a some info one of Amsoil's non-premium gear oils in a viscosity close to Ford's new recommendation:
Synthetic 80W-90 Gear Lube
Falex Procedure B (ASTM D3233) (failure load, lbf.) --- 2000
So taking a look at the specs it is instantly obvious that the 75w140 is much more viscous and can handle far more pressure than the 75w85. So why did Ford move to what appears to be a less robust lubricant for its newest Pony Car? I reached out to some engineers at Ford and here are the reasons they gave for the change; "75w85 axle lubricant has less parasitic losses (better fuel economy) and runs cooler than previous lubricants. The new lubricant is less viscous in cold weather. It is used on all 2015/16 Mustangs including the Torsen and limited slip applications."
So my initial assumptions were correct, much like the switch from 5w30 motor oil to 5w20 over a decade ago this change was driven by the goal of increased fuel economy. In this case, I agree with the change. For 99% of Mustang owners I don't believe they'll ever notice the difference, the 75w85 will be fine. Modern lubricants are far more robust than those from just 20 years ago, and for the vast majority I believe the new spec oil will meet all their needs. However, let's take a look at some Premium Synthetic options from Amsoil:
Amsoil Synthetic 80W-90 Gear Lube
Kinematic Viscosity @ 100°C, cSt (ASTM D445) --- 16.8
Kinematic Viscosity @ 40°C, cSt (ASTM D445) --- 109
Viscosity Index (ASTM D2270) --- 167
Falex Procedure B (ASTM D3233) (failure load, lbf.) --- >3500
Amsoil Severe Gear® 75W-140
Kinematic Viscosity @ 100°C, cSt (ASTM D 445) --- 27.5
Kinematic Viscosity @ 40°C, cst (ASTM D 445) --- 193.2
Viscosity Index (ASTM D 2270) --- 180
Falex Procedure B (ASTM D 3233) (failure load, lbf.) --- >3500
IMO, if you are adding significant amounts of power (150+ RWHP) to your S550 Mustang or using it in extended HPDE events I would suggest switching to a gear oil like Amsoil's Severe Gear 75W-140. You may loose 1/10 of a mile per gallon in fuel economy and 1 RWHP, but the added protection you would be giving your differential would more than offset those losses.
-SID297:beer: