Second, you can change the brand oil used any time, just continue to use the same high quality level recommended by the engine manufacturer. You can't mix brands of motor oil. Mixing brands is not the best practice because the additive systems in each may be different and when mixed may not function as efficiently as desired. However, no serious problems will occur if you must add a different brand of equivalent quality between oil changes. It is very important to maintain proper oil level.
Motor oils can cause sludge.
True, poor quality oils can contribute to sludge. Sludge deposits are formed by a combination of dirt, soot, partially burned fuel, oxidized motor oil, leaked coolant and condensed water vapor produced during combustion. These bad guys are usually present, and a good motor oil keeps them separated so they are drained with the oil and not deposited in the engine. Using a good quality oil and regular oil and filter changes will prevent sludge. Paraffinic base oils can cause wax buildup and sludge.