What is the consequence of topping a car up with a different grade of motor oil? My daughters 2003 focus tdi is running low and I have loads of 15/40 semi enhanced here. The car is not used for trans Sahara rallies etc.
is it only on the min ? top up wont matter on a ford lynx engine
I would suggest “avoid changing different viscosities too often”.
I switched from 20w/50 to 15w/40 and am now using 5w/30 which seems to last forever… My top-ups are months apart now, rather than twice a week with the thick stuff I used to use…
I’ve driven four different mondeos over the past decade, so there is an aspect of “Continuity” that makes such results relevant, I reckon. 
(The smaller the number, the more thin and runny the oil is. ("W"inter Running Temperature?) The two different numbers reflect on how runny the oil is at “Running cold” as opposed to “running hot”. )
You don’t use an oil where both numbers are the same, or it would struggle to turnover on a cold start (oil too thick to get to where it’s needed as lube…) OR as soon as the engine temperature got hot - the engine would wear far faster, and be more likely to seize on a long “City Drive” journey.
As said above 5W 30 is the recommended oil. A 15W will be thicker than a 5W at cold temps so will make the engine a little harder to start at low temps, but at operating temp the 40 is is going to give more protection at the expense of a minute increase in fuel consumption. In the “real world” youd not notice any of that though. And if only topping up youre only putting in about a fifth?? of the capacity between changes so youll actually be running a 7W 32* so there isnt any real difference there. Not enough of a difference to worry about I`d say.
*That assumes the old oil retains the “as new” original spec when old.
Which it doesn`t.
And don’t forget it’s a ford so will leak oil any way 
I should know I own 2 
blue estate:
And don’t forget it’s a ford so will leak oil any way
We’re not talking about the same car in two bits here are we? 
Its the Endura DE engine and these are very agricultural units. It’ll run on 15/40 no problem…
FoMoCo says its engines are more sensitive than most to oil viscosity.
5W refers to cold cranking resistance at Winter temperatures (the W stands for Winter, not weight), in sub-zero conditions 15W will present x3 the resistance of 5W (but luckily, it’s summer).
One thing to watch: diesel engines with particulate filters normally require a Low or Mid SAPS oil (refers to
the additive package, and stands for Sulphated Ash, Phosphorous and Sulphur). High SAPS oils will, particularly in an older engine that is using oil, form a metallic deposit on the filter element which is not removed by the active or passive regen processes.
A faulty or removed PM filter is now an MoT fail and potentially a C&U offence.
If the car didn’t have a PM filter as standard then this is not an issue.