Yes these are realistic concerns.
- the tiniest amount of rancid oil can make your dish taste bad.
- mold can grow on fat (e.g. penicillium in cheese).
- oil only has residual amount of water, so it will not accelerate most bacterial growth.
- shelf life for oil is about 6 months to a year in a dark cool cupboard
- shelf life is about a year and more in fridge (but the oil clouds and solidifies).
my suggestion would be that when your container is empty, just wash it with a natural degreaser (e.g. vinegar) instead of topping it off. Or top it off, but wash the smaller bottles every 6 months.
UPDATE
Be very careful about what kind of oil you use, in particular if it is not commercial-grade. In this other question you will see that oil or other canned products that provide anaerobic environment (=without oxygen) can be good conditions for some bacteria. Some of them are specifically bad. See also the wiki page for the C. Botulinum for an example.
Short answer: Homemade salad dressing, even with garlic, is generally considered safe, for a time frame of up to a week.
While some strains of botulism can grow slowly at refrigerator temperatures, in general botulism requires three things to grow in addition to having spores present:
- Low salt, low acid environment
- Low oxygen environment
- Temperatures above about 35 to 40 F (depending on the strain)
Garlic is one of the foods at risk for having botulism spores present.
Your recipe is vinegar based, and so is acidic. With the olive oil, the total dressing may be under the 3% threshold that completely inhibits botulism growth—especially when the dressing loses its emulsification and separates into layers, some layers may be less acidic.
Still, your strongest protection is the refrigeration, combined with the acid, and the short time frame.
Health Canada specifies that plain garlic and oil (much more at risk due to the lack of acid) can be kept in the refrigerator for up to a week.
You can therefore infer that with the additional acid, you will get at least the same week of safety, although I would hesitate to go longer as safety issues always warrant conservative assumptions.
Best Answer
The top of your fridge might tend to be a relatively warm spot in your kitchen: it's up high, and the fridge itself gives off some heat. Exactly how warm would depend on your kitchen and your fridge.
So it might not be the best spot for spices, since heat can make them age and lose flavor more quickly. Some other foods also prefer relatively cool storage.
Otherwise it should be totally fine; most things can handle being slightly warmer than the rest of the kitchen.