I grew up in the days of straight 30W oil for automobiles. When I bought my 1st Explorer in '91, it could use anything from 10W-30 to 20W-50, depending on the OAT. When we bought our '02 Explorer, only one oil weight was recommended; 5W-30. I think the Fords released since then use 5W-20, just like our 8's.

What's the reasoning behind the ever-decreasing oil weights? Less friction? Better mileage? Easier to crank in cold weather? I sometimes worry a little about such thin oil when driving across the desert when the outside air temperature is in the 100's or higher.

1.3L

 

The trend to lighter oils started with CAFE ratings. A lighter oil won't increase 1 car's mileage much but for 100's of thousands of cars even a slight improvement adds up.

I wouldn't worry too much about the desert temps. The car's got two big oil coolers.

 

They found that the straight weight oils caused excessive startup wear on particular types of engines. The multiweight oils allow quick flow to critical areas of the valvetrain, while still protecting under high stress high shear situations. Since something like 75% (don't know exact %) of wear occurs at startup...you can easily see where the benefit lies.

There are also more then one "viscosity" #. The 5w-20 (5 and 20) are "general" viscosity #'s which indicate the relative viscosity at certain temps. Generall this is the oils viscosity @40C and @100C. Then there is a viscosity index, and a HTHS #. The HTHS # is the ability of the oil to resist sheering under a high temp, high sheer situation (like in cam bearings). This number varies wildly from manufactuer to manufacturer, with some dino oils having very low numbers, to something like redline 5w-20 which has a HTHS # of almost a 40 wt oil.

http://www.bobistheoilguy.com/visc.html

There are vehicles on the market now, running V10's, in trucks, towing boats, (in florida's high heat, high humidity) with 0w-20 weight oils that are producing fantastic wear #'s. According to some older members, the engine should have seized .

Thicker is not necessarily better. Thats just an old worn out misconception. I think americans are just obsessed with "bigger # = better". What most people running thicker oils don't realize, is how close they are in viscosity to the lighter weight oils, and/or that they thin to a thinner weight oil after a very limited amount of time. Mobil1's 5w-30 weight oil is almost as thin as their 0w-20. While their 10w-30 is significantly thicker. Then you have something like 0w-30 "German" Castrol, which is thicker then most 40 wt oils.

If you feel like reading about oils and their technology, visit the bobistheoilguy forums. Tons of information, UOA's (used oil analysis), and even some information on rotary's with some rx8 uoa's on various oils.

http://theoildrop.server101.com/cgi/ultimatebb.cgi

 

Another point to consider (from a layman like me) is that of oil-flow rate vs. pressure. A heavier oil will build pressure, but won't flow as much as a lighter weight oil. Oil FLOW is very important, especially in the rotary engine where the oil is an integral part of cooling the engine.

In general, today's manufacturing tolerances are much better than in the "old days", so lighter oils are needed to get the appropriate flow through the tight bearing clearances, etc.

Today's oils are SO much better than the oils of years past, that I have no qualms what-so-ever about using the recommended oil grade in any vehicle today.