Switching to Synthetic....HELP!

   / Switching to Synthetic....HELP! #1  


Bronze Member
Nov 11, 2005
Montana 4940
I don't know that much about oil..(although I know a whole lot more than I did a couple of days ago thanks to this site.) Anyway, after reading several threads on this site, I am thinking that I want to switch to synthetic oil in my Montana 4940. My manual calls for 20w40 oil, but my dealer uses Rotella 15w40. I intend to use Royal Purple, or the new Rotella 5w40 synthetic.

I have 2 questions;

1. Do I need to use some sort of blended synthetic oil before I switch to totally synthetic?
2. Will the Rotella 5w-40 synthetic be ok to use despite the large difference in rating compared to the 20w-40 my manual calls for?

   / Switching to Synthetic....HELP! #2  
That 20w40 oil is a very unusual, at least I have not seen it. I have seen 20w50 and 15w40 and 10w40 but not the 20w40. Anyway, I have looked and looked at all the oils and the Rotella T syn 5w40 is a good rated oil. I like a little heavier in the summer 'cause it is so hot where I am. I personally use John Deere 15w40 plus 50 oil with oem filters.
No, you will not need to use a blended oil first, all the newer oils are compatible (synthetic and dino)and will usually say so on the container. Just be sure to use a good diesel rated oil and you should be fine. A lot of people swear by Amsoil 15w40 heavy duty diesel rated oil too. As for the manual, I suspect it is a misprint, but check again with the dealer and see what they say about that 20w40 rating. You can get a lot of information on a site called "Bob is the oil guy" if you really want to get more than you need to know about oil.
   / Switching to Synthetic....HELP!
  • Thread Starter
TPS..thanks for the advice. Yeah, the manuals for the Montana model tractors are lacking to say the least. I'm pretty sure some of the info was lost in translation.:) Thats about the only bad thing I've got to say about the Montana brand after 1000 hours though. My dealer has always used/told me to use 15w40 though. I might go ahead and look into the Amsoil or maybe Royal purple if I can find a 15w40.
   / Switching to Synthetic....HELP! #4  
Check out amsoil's site . they have a 15w 40 diesel
   / Switching to Synthetic....HELP! #5  
I'd use 5w40. You can buy these in diesel grade even at Walmart. The 5w is MUCH better for better lubrication at startup where most wear occurs, particularly to the top valve gear in cold weather. Even the 5w40 will be heavier than 40 grade at startup.

For my next oil change, I plan to switch to the Mobil 1 0w30. Been using 5w30, which is what is called for in the manual for my 4010.

   / Switching to Synthetic....HELP! #6  
If you contact Amsoil, either by phone or E-mail, they have engineers that will tell you exactly what to use for a particular application. And they are very good about answering any questions.
   / Switching to Synthetic....HELP! #7  
I don't think you can go wrong with Rotella 5w40 synthetic.
It will give you as much protection as you need. Regardless of all the available brands to choose from, in truth, most quality diesel rated oils will do what you want. Regular service and attention to detail is a key factor.
   / Switching to Synthetic....HELP! #8  
3RRL said:
... in truth, most quality diesel rated oils will do what you want. Regular service and attention to detail is a key factor.

This really is the important part of oil and lubricants. There is more expensive and fancier products out there, but the stuff rated to do the job, does exactly that. I think changing it more often is more important then what type of oil you use. With all the discusions on synthetic oil, I've never seen where it's worth the extra expense. Engines still wear out just as fast, or last just as long with the minimal priced, aproved oil as they do with the high tech, synthetics. Proper maintenance and timely oil changes are the only thing that you can do that will make a noticable difference in the life of your engine.

Kind of like going from a 40 to 80 hp tractor to pull the same impliments. It will do the job, but do you really need to spend the extra money on the 80hp tractor when the 40 hp tractor did the job just fine?

   / Switching to Synthetic....HELP!
  • Thread Starter
I spoke to the mechanic at my dealer today. I asked his opinion of synthetic oils. His opinion was that because synthetics swell more than dino oils do at higher temps, he did not like them. He stated that because the engine was broken in with dino oil, the synthetic oil may push the seals out a little, causing leaks. From some other things he said, I gathered that his experience was from a previous job, and some years earlier. I have heard that synthetic oils had this problem when they first were introduced, but that they have been "reformulated" to fix this problem.

Does any of this info sound accurate?
   / Switching to Synthetic....HELP! #10  
My experience is that new oils will rejuvenate seals, generally stop them from leaking.

Synthetic oils are fully compatible with dino oils. What synthetics are lacking are very many additives, because many of them are not needed.

I've switched to synthetic in quite a few air cooled engines, including an old Gravely. All my water cooled vehicles: 1 gas car, 1 diesel car, a gas pickup and diesel tractor, were switched to synthetic about 4 years ago. I put Mobil 1 in the tractor at 13 hours. The old Gravely went from using a little bit of oil to using none, but after the 2nd oil change on synthetic.

By the way, I think Rotella's "synthetic" may be just a hydrocracked dino oil. Switched to it one year on the old Benz and think I found this out. Switched next to 5w40 Mobil 1.

Next switch on all my vehicles will be to Mobil 1's new 0w30, except probably on the old Benz. Have some 5w40 left over for its change. It takes 7 or 8 quarts. For any makeup, I'll use the 0w30 that I buy for everything else.