Garage put some oil in car and after I checked the dipstick it looked black and thick. Car has since been running sluggish and the cause might be the oil(though this work followed an overheating situation so could be something else).
I did an oil change and managed to get 3.5l into the 4l capacity so that would mean .5l or 12.5% of old oil is still there. It sounded and moved a bit better after the change but is still quite sluggish.
In case that oil is the cause and the 12.5% oil can make that much difference, I need to get it out. However I do not know the viscosity of the oil they put in so doing constant oil changes may not help, since when draining the different oils will settle, not mix and hence the garage oil may settle in the area that doesn't drain out i.e. the 12.5% wont dilute and may locate in the non drain-able area. Therefore doing oil changes may not help.
Would engine oil flushes work for my situation? I've heard they can do more harm then good, any truth in this?