1qt every 4K is just fine, you will loose some past the rings and mist through the PCV, also some evaporates at operating temps
I like Mobil 1, for the most part the only true real synthetic that you can buy at your local store, it is generally cheaper than mail order synthetics (Amsoil Redline etc) and I have not seen any independent evidence that any of the synthetics outperform the others so I might as well use something available in every town USA
Picking the right viscosity is a very controversial subject, there are a lot of factors that decide what your motor needs, I received some basic education on oil in A&P school and a lot of reading on-line but I am not an expert by any means
Oil used to have just one viscosity “SAE 5, 10, 15, 20”, ect this told you how thick it is, oil viscosity changes widely with temperature, it is thicker when cold and thins when heated, a low viscosity oil is easier to pump (important in cold weather) and gets to parts faster on startup but unfortunately it does not protect parts as well as a thicker oil, as a thicker oil will take more pressure before it is squeezed out of the way allowing metal to metal contact, also thicker oil seals piston ring better, my piston engine instructor said “you want the thickest oil you can still pump under all circumstances” although this may not be fully accurate, if all you need is a 30wt to keep the engine parts separated why go thicker? Thicker oil besides being harder to pump stealing HP at the oil pump also generates more friction on moving parts robbing HP and generates more heat in bearings
Somewhere in there is the just right viscosity for you motor under the conditions you drive it in finding out what it is difficult
Then multi weight oils came around, through different techniques you could get oil that is more consistent with temperature changes, that is where the double number comes in, 10w-30 acts like a SAE 10 when cool but does not thin out as much when heated so it is more like a 30 at operating temperature
This ratio of how much it thins is called viscosity index, higher # is more thermally stable, this would lead you to believe that we all need to run 0w-60 an ultra stable oil but to get this wide swing in a mineral oil would require a huge does of additives and viscosity index improvers, VII’s are microscopic polymer coils that expand when heated and hold oil molecules together kind of like rebar in concrete to make it act like a thicker oil, but they have draw back’s, when pushed into a tight space like a crank bearing or cam lobe the coils line up like spoon in a drawer making them less effective, this is called temporary sheer, also they are fragile both mechanical action and high heat break them down and the oil will permanently sheer down to a lower viscosity, broken and burnt VII’s are a major ingredient in sludge
Synthetic base oil can be made more thermally stable requiring less or no VII’s depending on brand and weight one of their advantages
I would follow that Australian oil recommendation chart in the other thread, Mobil 1 tends to be at the low end of the viscosity range (ie their 30wt is almost a 20wt) , so if two different weights will work for your expected temps I would go with the thicker one
For weights like 5w40 and 0w40 that are not listed you can extrapolate the range they are good for
There are a lot of other things about viscosity like HTHS #’s and noak volatility but I don’t have a good enough grasp to explain them,
For more info search here for posts by Cary our resident oil guru, he has some excellent info
http://www.bobistheoilguy.com/visc.html
