gdhillon
Full Access Member
I recently put Rotella synthetic 5w40 in my truck, is the oil to thin for warmer temps? And how much does the oil being synthetic increase the change interval ?( with avg normal city driving)
You are thinking of Blackstone, but they don't do a separate soot test, only "insolubles" (which includes soot and other stuff). Wear Check, and others, (Google them) will do a separate soot check. Another sign of soot is when the oil begins to thicken. Usually oil gets thinner as the VII (Viscosity Index Improvers) wear out but it reaches a stage when the soot starts to make it thicken. Most HDEO (Heavy Duty Engine Oil) resist thinning pretty well, at least the ones I've tried (Royal Purple, Delo 400 and various Rotellas). Interestingly, the first few years with my truck, I used the oil type Ford recommended, a SF/CC 10W30, in a Pennzoil brand.
As to "5W" or "10W" being "too thin" it does not apply to warm oil, as clayton said. The 40 grading is the warm viscosity taken at 100C (about 212F), and 40 grade can be as thin as 12.5 cST (Centistokes) or as thick as 16.2 cSt at that temp. A wide range. The only cold viscosity rating is the 40C (100F) spec, which is not shown in the grade.
I base my vioscosity choices on my running oil temps, which in my engine are seldom if ever above 190F. Oil thins as it gets hot and thickens as it gets cold. At 190F, 40 grade oil is running in 50 grade territory or at the extreme end of 40, so it's really thicker than the engine needs. The Rotella 10W30 I commonly use, which is thick in it's grade, is running as a 40 grade at that temp (at the low end of it, anyway). Also, a 15W40 doesn't flow well even on a cool start (say 65 degrees). The saving grace for our engines is that wonderful oil cooler, which warms the engine oil up quickly as the coolant warms and keeps it pretty close to coolant temp as oil temp rises above coolant temp. By using a viscosity calculator (Google "Widman" with "viscosity" and a good one will come up in Spanish or English) and entering in the 40C viscosity and 100C viscosity (from Product Data Sheets for the oil), and the temp you want to see, you can tell what viscosity the oil is running at any given temp. Anyway, by monitoring my engine oil temp for several years, I concluded that the factory recommendation (at least in my '86 manual- see the chart which came in my original owners manual) is correct. It may not be for everyone, because it directly relates to oil temp and how the truck is operated. Thicker oil than you need can accelerate cold start wear, put the oil filter into bypass on a cold start (no oil filtration), put the oil filter into bypass at high rpms or when the oil isn't fully warmed up and it can decrease fuel economy due to fluid friction. Basically you need oil as thick as it needs to be at that moment. If it's thicker you are wasting energy. If too thin, it can cause wear so that's why you need the RIGHT viscosity for the situation, not some arbitrary choice that someone says is better. If you have the right viscosity, thicker isn't better or offers any less wear. As to my choice, I can only say for sure that it's right for my truck... and it was right for Ford in 1986 for trucks that fit the operating parameters. Oil has improved a lot since then but the one part of the old recommendations that is still valid are the basic viscosity recommendations.
You can improve on those recommendations in the "W"area. The "W" designation is a cold weather performance specification not a direct indicator of viscosity. The W grading breaks down into the following specs under SAE J300:
0W- Cold Cranking Simulator test (CCS) no more that 6,200 cP @ -35C (cP = Centipoise, another measurement of viscosity)
5W- CCS < (the "less than) sign) 6600 cP @ -30C = -22 F
10W- CCS< 7000 cP @ -25C = -13 F
15W- CCS < 7000 cP @ -20C = -4 F
20W- CCS < 9500 cP @ -15C = +5 F
There are also MRV Cold Pumping Viscosity (MRV=Mini-Rotorary Viscometer) specs and minimum viscosities involved in the J300 specs, but it all comes down to how cold pumping specs for the oil. By choosing the lowest "W" number, you can improve the cold flow of the oil as well as the "cool" flow... the first start on a 60-70 degree day for example.
Also, I have run 15w40 in my truck on occasion and when I did I checked the warm (180F) oil pressure vs 10W30 and there was only a 5 psi difference at 2000 rpm.
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