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[robson]

If I do an oil change with 10/40 is it ok to top up with 15/40 ( 5 litres in my garage)  or would it be better to put 15/40 in when I do the change and forget 10/40

[tigers_gonads]

i'd stick too 10/40 

[mathewst]

Only top it up in an emergency. in athor cases avoid doing it

[Del Boy]

Stick with the 10/40.

[tunnie]

believe they can mix ok, but personally i would keep to 10/40

[Cliffo B]

can't see any good for your motor so I'd stick with the same oil

[Kevin Wood]

I would imagine that the 15/40 is probably a mineral oil so not of the same standard as a semi synth like GM 10w40.

It will be fine for topping-up a car filled with semi-stynth, IMHO. It won't get added in sufficient quantity to make much difference.

If you were to fill a modern car 100% with it, I would bear in mind that it will have a shorter lifetime than a modern oil so adjust your oil change interval accordingly. Probably also best suited to use in the summer when it won't get too cold, given its' relatively high "winter" viscosity.

I would probably use it for topping up, for the lawn mower, or for when I come across a neglected car that needs a few short oil changes to clean the crud out.

Kevin