Chilly morning last week car would just about turn over.
Jump started, ran for a couple of minutes, then started OK.
Leave for 3 hours same problem.

Changed battery for new one, all OK for a couple of days, then from cold first turn of key laboured and would not start, second turn of key all OK.

Battery voltage around 12.5 volts, with engine running around 14 volts.

Could very sluggy (thick) engine oil be the issue?

Any better ideas please!!

Regards

If engine oil is that thick and sludgy its well past replacing

Get the engine toasty hot - add a chemical cleaner if you like - drop it replace filter and replace oil

Or
engine Hot - drop Oil
Fill with Cheap Asda Oil with a Chemical Flush
Run the engine till its had time to circulate properly
Drop the Oil again
Replace filter and refill with good clean oil

When Starting - is it struggling to turn over that badly?

poor turning over could be earthing
Get a Negative Jump lead - Clip one end onto the Engine Mount or the Bonnet Catch - and the other end on the Negative pole of the battery
try starting then - if it starts with ease - its an earthing issue on the Starter motor via the body earth

Also use a multi-meter and see how much of a drain you have

Ziggy

Thanks for your response.

The car belongs to my daughter, and she has not changed oil for ages, tut.

Checked for bad earth ((I'm an electronic tech).

Why I'm looking at the oil is because she said first time this morning engine just about turned over, the second time it turned normally and started.

I have since checked oil level which was low, so I have added a litre of "thin" general purpose engine oil.

I will now have to wait until tomorrow morning.

Adding thick to thin dont make it better
I bet its Black and stodgey - which means its getting hot as well

replace and Try then

Ziggy

I put the oil in to thin what was in the sump.

Started this morning first time.

I find it hard to believe "thick" oil could cause the issue, but maybe it did.

Now will flush the system and see how it goes.

Regards