The "weight" of any oil is determined by time through a metered orifice at a specific temperature. Conventional, Blended, and full synthetic. The only weight involved is the weight of the volume above the orifice. The cold test determines the lower number on multi-viscosity oils. A 20-50 oil flows like a 50 weight oil but has a cold test result of a conventional 20 weight oil. I have no idea what blending different "weight" oils might produce. I would also have no clue what would happen if you blend a Conventional oil with a Synthetic. Some labels want a complete change out. Drain oil, replace filter before you add their Synthetic to the engine.
