I certainly believe it's not ideal to mix oils. What are your thoughts on these quotes taken directly from their websites?
Originally Posted By: Amsoil
Mixing AMSOIL motor oils with other oils, however, will shorten the oil’s life expectancy and reduce the performance benefits. AMSOIL does not support extended drain intervals where oils have been mixed.
Originally Posted By: Mobil 1
the superior performance of Mobil 1 will be reduced by diluting it.
Originally Posted By: Pennzoil
It is advisable that you use the same oil for top-ups if needed, thereby giving you the best protection from the oil that you have chosen.