OK....calling all Oil Experts....WHAT WOULD YOU DO???

I need some help on making an “oil change†decision. I recently purchased a high mileage ’04 Prius. It had 175K miles on it when I purchased it, and I have put about 3000 miles on it so far. I was a little nervous buying a car with that many miles on it, but it appeared to be in excellent condition, and to be sure, before buying it, I took it to a dealership and had a certified Prius mechanic do an inspection. The mechanic found nothing wrong with it, and (in fact) said that it was probably in the best condition of any Prius that he had seen with that many miles.

My question is this….. I want to change the oil at this point, but even after reading for hours on the internet and this forum, I cannot decide what oil to use. Ultimately I would like to use a synthetic, like Mobil 1 5w30, but I am a little afraid of the stories on the internet that say that making a change to synthetic on a high mileage engine might cause the engine to start leaking or using oil. Right now, the Engine does not leak or use any oil. I have not had to put any oil in it in the 2 months and 3000 miles since I have had it.


The problem is this........ I do not know the history of this car. So I don’t know what kind of oil is presently in it. It might be Synthetic oil, but maybe not. I have read on Toyota’s web site where their position is that once you switch to Synthetic oil, you should not change back to dino oil. I guess that they believe that it could be detrimental to the engine to go back. In my research I have found many others who disagree with this stance....

Now I don't mind the part about not ever switching back to dino oil. Being on synthetic from now on would be just fine with me. But my hesitation is this..... I don’t want to switch to synthetic (if it is presently on dino oil) and risk starting an oil leak or oil usage situation. But on the other hand, maybe the least amount of risk is to just go ahead & use the synthetic….. because if I decide to use dino oil and it turns out that the engine has synthetic oil in it now, - and if Toyota is right and it could hurt the engine to switch back - (or in my case, would hurt the engine to use dino oil on the oil change) - well you see my dilemma..... seems like I am in a catch 22 situation since I don't know what kind of oil is in the engine right now..... and that maybe the least amount of risk is just to use synthetic....and hope that the engine has synthetic in it right now....If I knew for sure that the engine had regular (dino) oil right now, I would just stick with that and 3000 mile oil changes, and not risk causing (possible) oil leaks and/or oil usage on the engine by switching to synthetic. WHAT WOULD YOU DO?


Is there any way to tell what kind of oil is presently in the engine? Will an oil analysis be able to tell? Sorry for the long post, and thanks for your help. I have thoroughly enjoyed reading all of the interesting threads on PriusChat over the last 7 weeks and have learned a great deal since I bought the car. I absolutely love the car, but I don't get to drive it that much, because my wife loves it so much, and decided to start using it on her commute to work. So, I have to wait until she gets home from work, and then do my driving in the evening...

 

Once I decide what oil to use (synth or dino) I would go with a schedule accordingly. But if it is dino, I would probably stick with 3k intervals, and if synthetic, I would probably go with 5K intervals and hope that the synthetic would make it a little "easier" on my high mileage engine. There seems to be an agreement amongst those that I have read in various "oil discussions" that the synthetic oil is much better for the engines when the engine is "starting up", among other advantages.

But I doubt that I would extend the oil change beyond 5000 miles, even with synthetic. So maybe I should just go with regular oil. I know I would if it wasn't for Toyota's emphatic stand on the subject. And it seems like they won't elaborate. But since they won't budge on this issue, it just makes me think that they know some secret about their engines that we don't.

I'm sure that I am worrying about this decision too much. It's just that I am (forced) retired due to health reasons, and I really don't have the extra cash should I encounter an expensive engine repair.

Of course in buying a vehicle with 175K miles, I'm kind of taking that risk anyway. We just couldn't pass up the car, as it was in incredibly good condition and we bought it for $4800. (We spent 2 months looking at used Prius's before we found this one)

So purchasing this Prius, took us from driving a vehicle that was only getting 13mpg around town, to this Prius that is getting about 48mpg around town. It was getting over 50 mpg when we bought it, but once we put 2 new Ecopia tires on the front, the mpg dropped a little. Thanks for your input.

 

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Yeah, I know. We bought the car from a small car lot that was just starting out. Their story was that they bought the car from a local auto auction, and that it didn't have any service records with it.

But all of the fluids looked a little too perfect when we bought the vehicle. Like they had all just been changed. When you pulled the dipstick, their was absolutely no discoloration to the oil on the dipstick or the dipstick itself. There was no "burned smell" to the oil/dipstick. The coolant fluid also looked brand new.

So since the oil appeared to have been freshly changed, I emailed the owner of the car lot, and questioned him to see if he knew what kind of oil had been used. He said No, that they didn't have any of the service records. So I emailed him a second time to make sure, and once again, he said he didn't have a clue as to what oil was in the vehicle when we bought it....

After re-thinking everything, I think that you are right..... that I should just put conventional oil back in. I just have a feeling that the owner of that car lot, or the people at the auto auction where he bought it at, changed the oil, and I don't see either of those places spending the extra money on synthetic. So my guess is that it presently has regular oil in it, so I think that is what I have decided to do. And you have confirmed it for me.

So with the (probable) decision to stay with dino oil, should I use one of the "high mileage" conventional oils? I noticed that many of the "major label" oils do carry a 5w30 high mileage oil now. Or should I just use a straight 5w30 oil? I guess I am just not sure about all of these oils with all of these new additives. So, any suggestions as far as using a straight "5w30" vs "5w30 high mileage"? Or any other oils I should consider?

I have taken the oil fill cap off and looked inside, but it was in the daylight. I used a flashlight, and I could see that the actual internal parts where slightly discolored (slight brown). But when I reached in and rubbed some of the oil off with my finger, the oil was only slightly discolored (after 3000 miles) and I could not wipe any of the discoloration off of the internal parts. It just seemed like clean, slightly discolored metal. I will go look at it again tonight, with a strong flashlight.

 

Yeah.... I have googled that very subject..... many times, and you do get a lot of stories about people developing leaks after switching from dino to synthetic..... especially on high mileage engines. That's what makes me nervous. But according to Toyota, it is also not good to go from synthetic to dino. That is my problem..... I do not know what type of oil is in the engine.

Yes.... I had asked in my OP, if anyone thought that an oil analysis would tell me what kind of oil that I have, and according to xs650, an oil analysis, would not tell me...... I'm sure he is correct.....

So I am stuck with taking my best guess....... and that would be that the car dealer and/or the auto auction where the car dealer bought the car....... It is my gut feeling that neither of them would've spent the extra money to put synthetic in the car before they sold it........ so my best guess is that I presently have conventional oil in my engine.....