This seems to be a common question frequently asked here on the forums. Obviously when in doubt the first thing to check is your owner' manual or factory repair manual. As you will see it gives you a range of choices:
So what do those numbers mean? The type of oil most automobiles use is called a multi viscosity oil. Hence the two numbers on the label. The first number designates the viscosity of the oil cold and the second the viscosity of the oil at normal operating temperatures. Motor oil - Wikipedia, the free encyclopedia 5W30 is the recommended oil for best fuel economy and is what Subaru puts in the car from the factory. As you can see from the factory manual page above,5w30 basically works for most of us due to climate temps. Obviously some might run a different oil in the winter and summer seasons depending on where you live or by preferred choice.
But what should I use? Conventional or Synthetic? Or synthetic blend?The choice is up to you. There are some very good conventional motor oils on the market. As far as synthetic blends,it's kind of a scam. Either use conventional "dino" oil or your average Group III synthetic (it's basiclly a " synthetic blend anyways) I prefer synthetic due to longer oil change intervals,higher temp threshold (less chance of coking) and less sludge build up. Coking is a byproduct of oil when it breaks down. It is the solid like sludge that can block passage ways and drains.
"If the inside of the bearing housing resembles the bottom of a frying pan and is coated with black crusty deposits, oil coking was the cause of failure. The bearings are oil cooled, and during normal operation temperatures don't get hot enough to cause oil coking. But when the engine is shut off, temperatures can rise to 600 to 700 degrees F. inside the housing as the turbo undergoes a period of heat soak. The oil oxidizes and forms coke deposits in the housing that then act like an abrasive to wear the bearings. Using a high temperature "turbo" oil or synthetic oil, installing an auxiliary oil cooler, and changing the oil every 3,000 miles can avoid oil breakdown and coking problems. In water-cooled turbos, coking is less of a problem provided the oil is changed regularly and you use a quality motor oil. So if you find an accumulation of black crud inside the housing, better check the coolant hoses for a kink or restriction."
Turbocharger Diagnosis & Repair
As far as synthetics,not all are the same. There are generally three types found. Group IIIs,Group IVs and Group Vs.
Group III = Hydrocracked/hydroisomerized (mineral based "semi-sythetic")
Shell Rotella T synthetic
Castrol oil Syntec
Quaker state synthetic
Group IV = Polyalphaolefin (POA)
European Castrol oil Syntec
Group V = Ester based
Group IV and V are the "true synthetics". Above I just showed some examples to give you an idea. Hopefully this clears up some confusion and also make sure to use a quality filter when ever possible!My opinion of a quality filter would be: Purolator,Wix,K&N or BOSCH.
One of the best sites for oil and filter information!!
Good quick reads with lots of info
Motor Oil Viscosity Grades Explained in Layman's Terms
Car Bibles : The Engine Oil Bible