For anyone that runs it, why is the 5W30 listed as "New vehicle formula" on the bottle, whereas the 10W30 I've seen is listed as "Higher milage formula," yet many people run it on newer cars it seems??

 

I actually run Mobil1 5-30 in my p5.

The way this works is that engines are built to certain tolerances and clearances. The tighter and smaller these clearances are the better the engines run. However you need a thinner oil to maintain proper oil pressure. Too thick and oil cant get betweenthe parts...too thin and it doesnt stay in bvetween long enough to transfer heat.

The rule of thumb is : Always run the thinnest oil you can get away with. If you have to step up to a thicker oil do NOT go back to a thinner one. Once you have opened up those clearances running a thinner oil affects the pressure.

notice how ford/mazda reccommend 5w-20 now...as does honda...they are making their motors tighter...

 

The first 2 oil changes ever done on my car were at the dealer, and if I remember correctly, I think they used 10W30 dino oil, though my memory may be off....?

For my 3rd change, will it be safe to switch over to 5W30? I'll have 16000kms (~10000miles) by then....??

 

The 2002 FSM specifies SAE grade 10W-30 engine oil.

 

There is just so much mis-information about the differences between the different grades of oil that it is beyond a joke.

5w-30 has a "weight" specification (viscosity) of a 5W oil in the winter, and 10w-30 has a "weight" specification of a 10W oil in the winter.
Thus, the 5W-30 will be (very slightly) thinner at -20C. Big deal, unless you live in Siberia.

I suggest that unless temperatures regulary drop well below freezing, do as the service manual recommends and use 10w-30.

At operating temperatures, both 5w-30 and 10w-30 will be (about) the same viscosity.

The mobil "old engine" and "new engine" is just marketing rubbish.

Also note that as a 5w-30 has a greater viscosity range, more of its volume will consist of viscosity modifiers, and will probably not protect as well as the same brand 10w-30, although the difference will be insignificant in a Protege engine.

You can readily go from 10w-30 to 5w-30 and back again without problems.