All brand oils are miscible - no snot will do. Do not look at the base, you can give a calm synthesizer, add something that the quality (norm) is acceptable by the manufacturer, universal viscosity is 5W40 in our climate. If you have a semi-synthetic, then lower viscosity there rather than 10W40. We do not know in what condition the engine. Too high a viscosity and can burn oil, too low - if the engine is worn - it can finish off and can also drink oil.
On the other hand, complete replacement is the optimal solution, the more you don't know what is poured in there (the more synthetic additives, the better the washing properties, in turn, the more mineral base, the more frequent replacement is recommended to keep the engine clean, of course you select the quality class first, then viscosity, then base, but here I recommend a synthetic or good semi-synthetic).
