Thanks for the additional information and clarification about Tess.
Trimming the fur around her anal opening and vulva is definitely something I would continue to do.
Cleaning her with baby wipes is also a good way to keep the area clean.
Personally, I wouldn't use baby oil since it the oils are likely to cause more material to accumulate there. Warm water would be preferable.
You might also consider a waterless shampoo (LINK).
It's good that she's otherwise doing well; I don't consider old age to be a disease but if she's not currently taking any supplements or medications for her arthritis, I do have a few to suggest.
1. I’m a huge fan of joint supplements such as Dasaquin or Cosequin which are veterinary products specifically formulated for dogs. Quality control is a big problem with these products since what’s on the label is not necessarily what’s in the bottle which is why I mentioned specific brands. These are available on the internet.
2. Also, fish oil supplements can be helpful since they have anti-inflammatory properties. Welactin and 3V Derm Caps are good veterinary products and are also available on the internet.
It takes several weeks for these products to build up in the system, so you might not see immediate results. But once started, these supplements should be continued for them to be effective; you wouldn't stop and start them, in other words, like you would drugs...but you may already know this.
3. Another option would be Zeel which is a human combination homeopathic that can be used in conjunction with other NSAID medications and has a very low incidence of adverse effects Dose would be one tablet two to three times a day....I'd go with three times a day for the first 2 weeks, then drop back to twice a day for maintenance.
4. If she's not currently taking any non-steroidal anti-inflammatory medication and she's not vomiting, then Aspirin can be given at a dose of 10 mg/lb twice a day, with food to avoid stomach upset. Many dogs will benefit from this drug although often stronger medication is needed as arthritis progresses.
We often dispense pain medication, too, such as Tramadol or Gabapentin which can help improve the quality of life for many of these dogs.
Alternative therapies such as hydrotherapy, laser therapy, massage therapy and even acupuncture have been shown to be very useful for joint and spine problems.
I realize that I wandered off point just a bit in terms of the question you asked, but I hope this helps. Deb