Never a good idea to mix oil viscosities. When draining the oil you will never get all of the oil out. If you are not getting bluish smoke when you receive a little hard, 5w-40!would be better for fuel economy, to though most manuals will suggest 5w30 oil for better fuel economy (check your owner handbook for suitable oils).

“W” stands for “winter.” When oil grades are hyphenated (i.e. 10W-30), this indicates range of viscosity attainable with this particular oil, namely it behaves like 10 weight oil (less viscous) when it is cold or “winter” and thanks to viscosity modifiers (manufacturer added oil additives), it will behave like 30 weight oil (more viscous) when hot. So DONT MIX them as the oil will function differently with the 10W and the 5W and could be damaging to your engine, stick to one viscosity and the viscosity that is recommended for your vehicle, this is very important as manufacturers do allot of extreme testing with which viscosity is best for your engine e.g. in my case, I own a Citroen C4 e-HDi turbo diesel and Citroen recommends using only 5W-30 synthetic oil and not 5W-40 due to long term built up of contaminants (even though minute but its critical) for the longevity of your engine and damage to the diesel engine.

Its recommended and essential that you change your oil (especially on a "Turbo" and "High performance" vehicles) petrol or diesel) at max 10,000kms and that includes the oil filter! You should also use, every 2nd oil/filter change the Nulon "Engine Oil Flush" so that you don't put clean oil into a dirty engine as it also cleans your engine of potentially damaging contaminants.. Using synthetic oil is the best (for high performance, turbo, supercharged and LPG engines) and the only recommended practice from all major manufacturers, as you should never ever use mineral based oils as they will damage your engine on these types of engines! Hope this helps, as I've been doing this for a very long time and its worked for me!

The recommendation is 5w/30, but I would use 5w/40 Mobil 3000