I have a 2006 model, 318 se , 2 litre model , I've had it for 4 years now without issue its got 75000 miles and all serviced. I've decided to sell it on Autotrader to buy a newer one and someone viewed it last night

I've not used it for last 4 weeks and the prospective buyer noticed that on oil cap there's a bit of creamy stuff immediately he said. "" your head gaskets gone"". I'm not getting smoking etc and from what I can see there is no oil in coolant this model has no dipstick so can't check that

how do I know if it's just condensation or cylinder. Spoke to mechanic briefly he said he doubts it's the gasket and advised to drain oil and coolant.

Any other help. Please???

I doubt if its head gasket as well. Have you been doing a lot of short trips recently. This causes the condensation to build up. I would just clean the cap before anyone arrives to look at it.