Over time oil absorbs all sorts of things and after a while can't keep that stuff suspended. I'm in CA and we do lots of Central valley hot trips during the summers. Which case during the summer I generally change it at around 4500 to 5000 miles pending if we are hauling trailers etc.I have 6 mo but only 2500 mi on 5 qts of Mobil-1 (5/30). The 2008 Subaru Outback 2.5i 4 cyl w AT is driven daily on city streets and 4 hrs at interstate speeds (70-80mph) every 60 days. The local driving is in 20-30 min range at stop and go speeds. Temps are central Cal moderate, but very hot in summer.
Is this severe enough service to warrant changing synthetic oil by the calendar vs actual mileage? It somehow seems nuts to change at 2500 mi when the factory says 7500, especially with synthetic.
Thanks for your comments.
Regarding time vs miles 6 months for a single change is a drop in the bucket regarding vehicle cost yes change the oil. We have a large SUV the family 3rd car which sees about 6000 miles a year I try to change the oil no later than 6 months usually around 5 months. Synthetic is better at staying stable and clean that long but some would argue that the impurities sitting in the oil that long could cause issues with gaskets and seals.
I wouldn't go more than 6 months and during the hot dusty summer months no more than the 3-4 month time frame. Central valley is hard on oil due to the heat and lots of dust.