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Read our primer articles on High Mileage Oil, Synthetic Oil and Kinematic Viscosity

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why would people use gas motor oil in a diesel ?

turn that around and there is why diesel oil is best for diesel motors .

unless someone wants to try to keep the soot particles that is not in a gasoline crankcase suspended in a gasoline engine for 30,000 miles like the trucks do with mineral oil which would be sillyness to try .

oil gets dirty and needs to be drained . no need for hi base number in a gasoline motor using mineral oil.
On sludge prone gassers or high mileage ones, diesel engine oil with its cleaner running ability works out well, also the shear strength of diesel oils are superior to their gasoline counterparts, I use multi fleet Delo 400 on my gas, diesel and motorcycle as well as gensets, works out quite well. My RC45 runs quite well on Delo 400, better than expensive Motul.
VIIs lend themselves to different applications, depending on their particular Shear Stability Index:

The 50 SSI VIIs are most frequently used in Passenger Car Engine Oil (PCEO) applications when fuel economy attributes are critical.

The 21-35 SSI VIIs are used mostly for heavy duty diesel engine oils and other applications with more stringent viscosity retention requirements.
stringent viscosity retention requirements ?

Probably why they load those oils up with so much VII's . To last the interval of 20-30k miles . It's sure not an ester or PAO thats keeping it from shearing with these mineral 40wts

They stay ingrade in pick-up truck diesel cause they run like 5-6k miles . Big sumps and low low rpm compared to cars .

I can't see the reason for it in modern OHC gas engines. Stuff is thick like glue at 30f no matter what the 15w part leads to believe and can do harm depending . 10w-30 counterpart not all that much better at same ambient .

Even Redline recommends against using their diesel oil in gas engines .

Consumers always right Razz
Haven't seen Delvac or Delo thicken in the gasser or my RC45, the RC45 engine happily revs to over 12000 rpm, have 14000 odd miles on it, engine behaves and sounds like new, after initial use of Motul synthetic, switched to Delo 400 and it has done 10,000 miles exclusively on Delo. There are oil analysis reports on HDEO like Delvac and Delo with gas engines, majority if not all of them have come out favorable.
Thickening is not the problem with bikes . Shearing is and those oils will shear to a 30wt in as little as 1k miles . Seen it time and time again . Even cam chain bikes like the 929-954 murder those oils and if you truly do have a RC45 , you might want to run an analysis to see what all those cam drive gears are doing to your oil .

Unneeded stress on oil pumps in cold ambient that can lead to premature pump wear and eventually oil pressure loss , more fuel used during the warm up phase , hydraulic lifters engineered for 30wts and below that don't operate will full efficiency with incorrect oil , higher cylinder wear from cold starts and a number of other things are some of the likes oil analysis cannot pick up that leads people to believe these 15w-40's do better than lower VI PCMO's in modern gas engines . There's more to it than meets the eye . Alot more .

Even the 5w-40 diesel oils are a bit too much for most automotive gas engines . They are around 100-110 @ 40c whereas a nice PCMO 5w-40 will come in around 75-85 @ 40c .

Motorcycles in hard use will smitherine those diesel synthetic 5w-40's also .

This is a never ending debate . The thicker is better thing ... the doping of well built oil with additives ... ahhh forget it . Have other things to do Smile
Last time I checked I did have the RC 45 Wink, don't know why you would respond with a statement like that, the fact that the engine still has original compression speaks of the quality of the oil. As for shear strength, a HDEO will wipe the floor of a regular PCMO, as for analysis, hate to mention BITOG but afraid, I would suggest you take a look at the analysis numbers posted by motorcycle users using HDEO, majority of them are refugees from expensive synthetic motorcycle oils, they found better performance and wear numbers using HDEO, I too used Motul synthetic initially and also Mobil-I 15W-50 on my RC45, the feel was just not good, the shifting was poor, the engine was noisier as compared to now using HDEO.

I have mentioned earlier in this forum that oil analysis is not possible for me here, wish I had it done while my RC45 was in US, the labs here only accept commercial orders and individuals are not catered to.

I use HDEO on my gas cars and gensets, all of them have their internals clean, the gassers run excellent, one is approaching the 100,000 mile mark and is in excellent condition. It is indeed a never ending debate, but what works in real field testing is what I am interested in, pass the 50C weather test and you got me as a customer for life.

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