I have painted thousands of residences and buildings including hundreds of multi unit properties and even water towers. Not to mention the hundreds oh homes I have flipped for landlords and property managers. I have painted in in 6 states and at one time had as many as 16 full time employees. Trust me when I say I know what I am talking about. True not all colors and paints will cover every color and paint. However it is not unheard for a paint in a light color to cover a paint in a dark color. Where a product like kilz is great at what it does, it is not necessarily needed to cover up an old color although certainly makes it easier to do so. Oil based paints are not the thing of the past nor ever will be. Are you sure your not thinking of lead based? Oil based paints and primers have a specific purpose one of which is, oil base is needed to prime certain things latex cannot, and another is oil is needed in winter times to be used outside where latex cannot. They still sell many oil based paints. However it is true that oil based paints are not used nearly as much as previously used.