It’s natural to check oil levels under the hood, but if you have a rear-drive car, you should periodically check the oil level in the rear-axle differential, adding or changing the oil if necessary. Car manufacturers do not presently recommend routine differential oil changes, and many do not provide drain plugs. But they always provide filler holes (sealed by plugs) to add oil, a small amount of which may be lost from evaporation and seepage.
To check the differential oil level, jack up the car and support it on safety stands at all four wheels. This is necessary so that the car (and the oil in the differential) will be level.
Remove the filler-hole plug. On most cars it is a threaded plug, but on Chrysler products it may be a rubber plug that you pull out and push back in.
Ideally, with the plug out, oil should ooze out at the bottom of the hole. Within 1/4 inch of the hole is an acceptable level. If lower, add oil until it is even with the bottom of the hole. If the oil level is very low, check the differential housing for a leak.
Install a gear oil marked “For service GL-5,” with a viscosity in the range of 80-90 or 85W-90. Use a funnel with a hose attached or a clean household squeeze bottle.
If you drive your car into water deep enough to submerge the rear axle, change the differential oil shortly afterwards. If there is no drain plug, use a siphon pump to draw out the oil (and any water) from the filler hole. Then add fresh oil.