I have been googling and searching and reading on here as well. I have a 2002 Prius with over 100k, past records show Valvoline 5w30 at around 3100-3300miles faithfully.. I want to switch to Mobil 1 synthetic due to the positive comments on here plus with synthetic I won't have to change oil as often. So what do I go with? 0w-20 0w-30 5-30 etc? My driving habits: Southern Indiana. Winters are typically no colder than low 20's to upper teens. Summers are 80-95 and HUMID!!! I drive about 2-2500miles a month, Usually at least 30 cold start trips of less than 5 miles each a week. (Deliver pizzas as a second job.) About 70% of the miles are in town no speeds over 35mph. and usually 4-5 stop and go sessions per 5 mile trip.
I dont know about you year but I know for the 04+ the manual suggests 5w-30 but I have heard on this forum that toyota now suggests 0w-30.
I assume your Gen-I is same as my Gen-II which is 5W-30. If for example you go with Mobil-1 you have choice of 5W-30 or you can opt for Mobil-1 0W-30 Advanced Fuel Economy, which states on the label it is acceptable for 5w-30. I have the 0W-30 in right now for better MPG in winter.
Here is a Toyota oil chart, it says for your 2002 Prius you can use 5W20 or 5W30. Living where you do I would uses either 5W20 for winter and 5W30 for summer or 5W30 all year around. The chart only recommends 0W20 for 2004 and newer.
I like the chart but it is hard to interpret. On this site, never heard anyone say anything other than 5W-30 is official recommedation for Gen-II in USA. I know some try other grades and in other countries they have other recommendations.
I think 5W30 would be a good choice, that's what I used in my '05 all year around. I also checked the Mobil 1 site and they recommend Mobil 1 5W30 Extended Performance for your Prius. If you decide to go with a full synthetic oil you could easily get by with an oil and filter change every 10K miles.
...well like minds then, I have the Mobil-1 5W-30 extended performance already purchased ready to go in at 105k (or later if I procrastinate).
