I suspect Mercedes got it really right with their rear cv joints. If you do not drive them too much under lubricated. It seems adequate to just add the oil. In general they may be the toughest part in the car. Rather than using an oil recirculation system but grease instead. I do not believe they could have both lasted the distance or time. For all practical purposes they rate of wear if kept heavy oil lubricated is virtually nothing.

As with many things in life some subjectivity has to be applied. They have to be running for a time in a semi lubricated condition until they are audiable in some fashion. This implication to me is some wear has occurred. How much will vary with circumstances. Yet if they settle down with new lubrication I consider it not is an indicator they need changed.

There is even a too much wear indicator may be present in this type of scenario. If the noise is there constantly all the time. To me running then too dry has been left too long. Taking time to warm them up before the noise occurs to me is an indicator there is not much existing clearance yet or they would not be capable of binding. After they warmed up a little. since your noise starts in about five minutes. I would suspect yours are still pretty unworn. Just do not put off getting some heavy oil injected.

With the thermal expansion of the friction heat. Noise from the first use of the car and constant in nature. I would change them. These joins if properly relubricated say every five years might go an indefinite amount of milage. Well beyond the rest of the car surviving is a strong possibility. Anyways I just add a little heavy oil before one of these cars goes into service. Anything is better than dealing with a road breakdown today otherwise.

I also have no ideal of where the oil migrates to over the years. Probably a tiny amount gets by a seal with time. Another thing or reason they should be on a maintenance list. The original axels with their high quality are really expensive. Plus any road breakdown can cost real money and time before you are mobile again.

Driving a thirty to forty year old vehicle is just a little more demanding to keep reliable. Actually with some effort and time this is achievable in my opinion on these cars. .If the accumulated milage is not really excessive. At reasonable cost and effort. Actually at far cheaper costs than one would expect.

Three hundred thousand is not too high for the rear cv joints. In my opinion. Tragically I never believe the odometers unless there are some indications they might be right are present though. Even then it is a tough call at least for me. Instinctivly that you want to believe what you see to me is human. Described as the wearing of rose colored glasses usually I have found pretty accurate.