I always thought synthetic oil is better because it is thin at room temperature so that it protects the engine better during a cold startup. This could cause leaks until the engine warms and the normal operating tolerances are achieved. Does this make sense?

Yes, I agree. Cold startups are to synthetic's advantage.

Synthetic is better oil, in just about every way--depending on the oil. In general, it lubricates better, it holds up better to high heat and mechanical wear, and has less stuff in it that can build up gunk.

However, it seems like it is too slippery for easy engine break-in, so most people recommend using conventional oil to break in a fresh engine. It also can wash away built-up gunk on old engines, which can start leaks where there hadn't been any before. But the risks of that are evidently far less than they used to be.

It is usually more expensive than conventional oil. Could be enough to make a difference.

Current oil formulations (both synthetic and conventional) are formulated to be friendly to emissions equipment but not so friendly to old flat-tappet engines like ours. Some of the oils which have the correct additives for our engines (like Brad Penn, or the "off-road only" Valvoline VR-1) are only really available in conventional oils. Others may be available in synthetic.

I use one of those two oils in my 914. If I could find a synthetic oil with the right additives that had proven to help keep our engines alive, I'd probably switch to that. And I'd definitely switch my wife's 911SC to it.

--DD

Dave - You must be aware that VR-1 is available as a synthetic and Brad Penn is available as a synth-blend.