Engine oil or something else in engine coolant - gti

[Bpgti]

I don't know if anyone asked about this. I check regularly my engine oil after operating temps are reached and coolant level when engine is cold but have noticed something different today. Had an eng oil top up at service centre today while I was there for the new fix and I found the colour of engine oil is like much darker - deep black in colour compared to a couple of days ago when I looked. It was kind of light brown and looked more watery. Now it's much more thick and black in colour. car has done 5350km. Could it be it was topped up with synthetic oil. Does it affect the engine if it gets mixed with mineral oil? Also noticed something like a big bubble in the coolant tank which seems to be there and not move. It makes that part of coolant tank shaded - same when hot or cold. What worries me is there was oil on edge of the tank which might have been spilled but since I never requested top up on engine coolant, tank cap shouldn't have been opened. But what is it?? I've been looking at coolant level at least a couple of times everyday and haven't noticed it before.

[Greg Roles]

Hmmmm that is a bit of a worry, oil always ends up black obviously but the coolant situation is a bit of a red flag....in your shoes with a new car I'd be straight back to the dealer and get it in writing that it is OK, which will force them to investigate it!

[Bpgti]

Hmmmm that is a bit of a worry, oil always ends up black obviously but the coolant situation is a bit of a red flag....in your shoes with a new car I'd be straight back to the dealer and get it in writing that it is OK, which will force them to investigate it!

Thanks for the heads up. Wouldn't something like oil float in coolant? The bubble or shade I was talking about was initially sort of at the bottom and are large in size along the same level in the tank. there are a couple of them which are seenacross the inside of tank - I've taken photos when the car was cool last night and now it seems to have moved up a bit while the eng coolant level is same. Btw engine oil has always been the same colour every time I checked it - light watery brown colour. What happens when mineral oil is mixed with synthetic one?