I learned 35 years ago that a rear axle differential uses extreme-pressure lubricant that becomes more effective as it binds with the gear surfaces, and that regular replacement of this fluid is unneccessary. Is this still the case? My 20-yo Volvo has 155 k miles. Other than the first service, the manual doesn’t schedule any changes for the non-turbo version, but calls for relacement every 10k miles for the turbo.
Differential lubrication was usually just topped off and changed only if there was a mechanical failure in years past. Now most vehicle manufacturers add the rear differential to the maintenance schedule. The schedule varies from maker to maker so always follow the owners manual recommendation for any normal or severe service. Most are in the 10-40K range but some may be less than 10K or more than 40K.