What's the difference between car and heavy truck gear oils?

[Jimmy_Russells]

Between "car" based gear oils like Mobil 1 or Pennzoil Platinum, and "truck" based gear oils like Delvac 1 and Spirax?

Is it simply marketing and approvals? They all seem to be GL5. My brain wants to think the truck based ones are for slower speeds and heavier loads and may not do as well at high speeds (foaming or similar), but I really have no idea.

 

[dnewton3]

Yes- pretty much marketing and approvals, relative to light or heavy duty applications.
Many gear lubes, including some you mention, are on the SAE J-2360 QPL list.
Both "car" and "heavy truck" lubes are tested to the same standards in that qualification process.