The whole never switch back and forth is an old Oil Company and sleezy mechanic trick...of course they want you to switch to synthetic and stay with it...$$$...my GF mother was a chemist for exon...(has patents for testing oil in the field...ie she knows her ****) she said you could mix them all together and it wouldn't make a difference... you don't want to even know what she says about different octane gas...but keep pumping that 93 helps the stock price..

Valvoline would agree and even put it in writing

Valvoline.com > FAQs > Motor Oil Car FAQs > Full-Synthetic Motor Oil

 

For what it's worth, and I live in Washington state, My engine had Castrol 5w30 synthetic in it for 45k miles and it ate the camshaft. The dealer made this choice as I had an extended warrantee up to that point. of course the camshaft ate just after that ended,

After getting a new shaft/lifters/oil pump/timing chain/front cover/etc I will never go synthetic or 5w again.

I run Rotella 15w40 year round with no cold start issues.

 

http://www.bobistheoilguy.com/





lots of good info..........

 

I had a 2006 Sprinter, it came with Mobil 1 0W-40 from the factory, oil changes were recommended every 15k.
I had them done at the dealer every 10k.
At 162,change the engine let loose without warning.
$12,000 to replace the engine with a rebuilt one.
Started using Rotella 15w-40 and changing it every 5,000 miles.
I will NEVER use Mobil 1 anything ever again.

I use Rotella, 15w-40 in the summer, 5w-40 in the winter, change it every 5,000 miles no matter which one I am using.
Consider this, the OEM oil change interval is 7,500 miles with a dino oil, so why waste time and money changing it every 3,000 miles?
Thats more big oil lies to get your money.
I know several mechanics who use a full synthetic oil, like Valvoline and change their oil every 10,000 miles, they say 5-6,000 with a dino oil.
The reason they go every 10k is because it is easy to remember, same reason I use the 5k rule, easy to remember.
I am currently at 218,change, next oil change is at 220,change, and then 225,change...easy.
So for them it would be 220,change, and then 230,change...again easy.