I was funning 10-40W (diesel) in my 75 115 240D (motor is an early 123 240D one). I was thinking maybe it's on the thin side for summer (in UK). I don't know much about oil ratings, and was thinking 10-40 meant it was 40W when cold and 10W when hot. So I thought straight 30W would be 30 the whole time...

I put it in and my oil consumption is now pretty bad with some pretty solid blowby. So I'm now thinking my understanding of oils is wrong. Can someone explain please?

But my real quetsion is, what's a good summer oil for a 240D in the UK? (temps up 30 maybe, not much and never more). I don't tow, and mainly motorway driving. I was thinking straight 40W, but there is a 20-50W diesel as well. I assume it's important to stick to diesel oil etc.?

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1975 W115 240D Mercedes-Benz
1950 SI Land Rover
1963 SIIA SWB Land Rover
1945 C15A Chevrolet CMP
1981 HJ47 Toyota Landcruiser