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In general, yes. Mixing oil brands, types (synthetic, semi-synthetic and mineral) and viscosities is fine. For example, you can use a Motul oil to top-up your engine oil when another brand was used for the oil change. Clearly you want to use an oil of the right viscosity and specification, but in terms of oils mixing, the only problem is that the quality of the better oil is diluted by the lesser one. There are a few exceptions; Castor and plant based shouldn't be mixed with conventional oils. 

The life of the oil is dependent on many factors.

Full synthetic oils last longer than semi-synthetics or mineral oils, so although they may cost more in the first place, a full synthetic can work out as a cheaper option in the log run. Many cars specify the use of full synthetic long-life oils and these may last over 20000 miles or up to 24 months.

If the car is used on track, the oil is subjected to far harsher conditions than motorway use. That may mean that a good track oil will have broken down sufficiently to need changing after 10 hours of use, whereas the same oil would be good for over 100 hours of use on motorways. Short journeys are very hard on the oil as it does not get the chance to get warm and flow properly as well as acceleration and deceleration making the engine work harder. Motorway use is the easiest condition for oil, the speeds are fairly consistent and rarely push the engine hard, there is plenty of air flow to help cool the engine and the oil has a chance to get up to temperature and flow properly.

Certain engines suffer from fuel dilution and that is one of the quickest ways that an oil can breakdown.