Have got alot a concerns - 96 855 N/A

96 855 N/A auto
146,xxx miles

Concern 1. Im using amsoil 5w30 and it seem to be consuming it as there are not any leaks. I dont no what type (conventional oil/synthetic) of oil was used at the volvo dealer were it was serviced. Should i switch to conventonal or blended oil?

Concern 2. After doing the first oil change i (bought car at 140,xxx) later found milky stuff on the dipstick. Did not smell of coolant and the level was not low . could this be related to the switch from conventional oil to full synthetic? (concern 1)

Concern 3. Bought the car aware of two codes EFI 436 and 425. checked for a vacuum leak, could not hear or see a leak. With the current milage should just replace the front o2 sensor, or look for another cause. The reson being the sensor costs at least $147 for IPD.

Concern 4. I commute from wisconsin the cities in minnesota for school and work. These trips range from 60 to 130 mile round trips. The best mpg I have been able to muster is 26mpg going 65-70mph with little stop and go traffic. What type of numbers are you guys gittin. Im hoping this is due to a faulty o2 sensor in concern 3.

Thanks in advance for any help or ideas. If more info is needed just ask.

Thanks,
John