My "aging" dogs are fed TasteOfTheWild kibble topped with a bit of wet food (variety of good brands) or a raw medallion (duck or pheasant) and with a fair amount of water so its kind of soupy (vet suggestion). Their weight hasn't changed and my vet is very happy with their condition. He also does a CBC every year on them for monitoring purposes, which I'm happy about.
I'm real skittish now with essential oil usage. A couple of friends who are into that got me started using them on my dogs but mine started having bad reactions (Teddy's allergies went berserk and he scratched off a lot of fur before I got it under control again). I do use a botanical flea/tick spray but even there one has to be very careful - I tried one that had cedar and cinnamon oil in it and it caused weird shedding and scratching.
While some veterinarians will allow the use of lavender oil for certain issues; NOT ALL oils are safe for dogs; whether they are diluted or not.
If a dog has sensitive skin, clove oil and cinnamon bark oil have been demonstrated to cause dermal irritation; similarly cedarwood and lemongrass (common ingredients in herbal flea sprays) can cause irritation.
Be careful, particularly if you dog already has allergies, when using essential oils - some have benefit but others don't.
If you want to use essential oils, consult a holistic vet first.