Oil weight, Does it makes that much of a difference? Options

[914itis]

On my six, I was using joe Gibbs 10x40, I recently switched to Kendall titanium 20x50, with added zinc
For the first time I drive the car after the oil change and my temp surged 20 degrees higher. It was not even that hot today.

Is it the oil weight or the type?

Correction : Kendall titanium high performance 20-50

[somd914]

in my 3.2 911 I run 20-50 as per the owners manual it is rated from 0-100 deg F.

My temps typically stay around 190-200.

Granted, cooling between a 911 and 914 are different and I don't know which engine you have.

As for multi weight oils, first number is the viscosity when cold, second number is the equivalent single weight viscosity at 100 deg C. Thus 20-50 cold has the viscosity of single weight 20, thinner than single weight 50 when cold. But at 100 deg C it has the same viscosity of what single weight 50 would be, thus thicker than single weight 20 when hot.

Personally I can't see a change from 10-40 to 20-50 causing a 20 deg increase, but I'll let the more knowledgeable chime in.