Oil change

nm9stheham said

FWIW.... Just be aware that a 15W50 full synthetic at cold temps (like below 40F) is going to be as thin as a 5W dino oil at those colder temps. The 'cold' temp viscosity of motor oils is measured at 104F and the oil's behavior at much colder temps can be quite different depending on the oil material. Pout them both out of the bottle at room temp and the lower viscosity of the full synthetic will be immediately obvious.

So a 15Wxx Mobil1 is gonna work OK for most street engines, even at cold temps.

And yes, the old stock spring pressures seem to not be an issue with the now-lower ZDDP levels. From several research articles read, the ZDDP gets 'activated' when pressures on the microscopic 'peaks' gets past a certain level, and the less the spring pressure, the less ZDDP gets used or needed. I personally will try to stick with the older, higher levels regardless.

Click to expand...

Sorry, but with all due respect, that's incorrect.

The first number that is the prefix for the "W" is the grade assigned to the oil based on it's flow properties at 0°C, or 32°F, and the second number represents the flow properties at 100°C or 210°F. Or, in other words, the freezing and boiling points of water.

A 15W oil, regardless of it's base stock, will flow in the range specified for it at 0°C, and not out of it. Doing so would mean it is not a 15W, but rather the oil would be assigned the grade number that is defined as that which flows as it actually did.

Synthetics generally have a higher viscosity index than dino base oils, which means they are more resistant to thinning when heated. That means that less additive, such as viscosity index improver or pour point depressant, can be used to make it perform and stay in spec. When additives are consumed, that's a way we get to the situation Castrol likes to describe as "breakdown."

Your assessment of how ZDDP works, however, is spot on as I understand it.