Motor Oil Type
First and foremost, do not overlook the fact that viscosity is not the only criteria critical to proper motor oil selection. You must choose oil that is matched to your application with regards to its formulation and design, specifically base stocks and additives used. For example, if you are running a diesel engine, then you had better be sure the oil you choose is rated for diesel engines. Similarly, if you are running a gas engine, then the oil should be formulated for use in gasoline engines.
Of course, sometimes, an oil can be rated for both gas and diesel use. However, your oil should meet at least the minimum API specifications required for YOUR vehicle. For instance, if your vehicle manual calls for an API SL oil (which is a gas engine oil specification), then your oil should specify that it meets/exceeds that minimum standard (in other words, is should be rated for API SL, SM,...). It MAY meet other specifications as well, but it MUST, at a minimum, meet the API SL specification.
Likewise, if you are running a diesel engine and your owner's manual specifies that you should only use API CI-4 oils, then you should not use an oil that is only rated for CH-4. This is not a recent enough specification. However, if the oil you are considering indicates that it meets API CJ specs, then you're ok, since "J" comes after "I". API specs are generally "backwards compatible", meaning that the CJ spec also is appropriate for use in vehicles calling for CI, CH, CG, CF, etc. (all PREVIOUS API classifications).
Moreover, some vehicle manufacturers will specify OEM specific ratings that an oil must also meet. This is more common with European vehicles, but is becoming more common with US manufacturers as well. So, if your owner's manual says that your oil is to meet BMW specification XYZ123, then you had better make sure that the motor oil you're considering indicates that it meets that OEM spec.
Synthetic or Petroleum
This is a question that is actually a relatively critical one IF you have any concern over how long your vehicle/engine should last. There are plenty of situations that do NOT warrant the use of synthetic oil, and you may very well be in one of those situations, depending upon your vehicle, how you drive, where you drive or how long you intend to keep your vehicle. Of course, it is just as likely that your situation may very well warrant the use of synthetic oil, and the decision regarding which you should choose is not necessarily a simple one.
The truth is, there are MANY variables that affect this decision, not the least of which is just how much trust you put in a synthetic oil to offer enhanced protection and possibly extended oil drains. Some of the many benefits that synthetic oils offer can only be taken full advantage of if you actually trust them to offer those benefits and push the oils beyond what conventional petroleum oil could accomplish.
The Viscosity Question
In most cases you can simply go with the manufacturer's recommended viscosity grade. Of course, if you're considering a synthetic oil, it is generally very safe to go to a lesser "w" rating than what is recommended by the manufacturer (although the higher the quality of the synthetic, the more true that statement is). For instance, if the manufacturer recommends a 10w30 viscosity (and isn't already specifying the use of synthetic oil), then you would be safe to use a 5w30 or 0w30 synthetic oil.
Generally (see below for exceptions), if the vehicle manufacturer specifies an XwY viscosity oil (such as 5w30, where "X" is "5″ and "Y" is "30″), under most circumstances you should stick with the same "Y" value, but can go to a lesser "X" value to improve cold weather performance and possibly fuel efficiency, but, in my opinion, ONLY if you're moving to a synthetic oil and synthetic is not already being specified by the vehicle manufacturer.
In other words, if your vehicle manufacturer is calling for a 10w30 SYNTHETIC oil, then, although you MAY be able to go to a 5w30 synthetic oil, I, personally, would not recommend it without some significant investigation into the issue. On the other hand, in nearly all cases, if the vehicle manufacturer is calling for 10w30 oil and does NOT specifically indicate that you should use synthetic oil, then, you should be able to EASILY move to 5w30 synthetic oil, and possibly even 0w30 synthetic oil with no trouble at all.
If the manufacturer specifies multiple viscosities that can be used in your vehicle, if the manufacturer does not specify the need to use synthetic oil, then a petroleum oil of ANY of those listed viscosities would be fine, although, the conditions under which the vehicle will be driven would dictate whether to stay at the "high end" or "low end" of the viscosity range listed.
Will you be driving in cold temperatures mostly? Congested city driving where engine temps can rise considerably? Heavy towing (same heat issues)? Do you live in a very hot climate? All of these and other driving conditions should be taken into account in trying to find the BEST viscosity for your application. However, ANY of the listed viscosities should be adequate.
In contrast, if the vehicle OEM is not specifically calling for synthetic, but you're planning on going that direction, then, nearly always, you can safely stick to the lowest "Y" value listed and could even go to a LOWER "X" value. So, for instance, if the manufacturer said you could use 10w30 or 10w40 in your vehicle, unless your application falls under one of the exceptions below, you could safely stick to an Xw30 oil (using the lowest "Y" value listed) and you could go with a 0w30 or 5w30 oil safely, even though the manufacturer listed only 10w options.
Exceptions to the Above Rule
1) If yours is a racing application, more investigation would be necessary to determine the proper viscosity to be using in your engine? 2) If yours is a heavy duty application where your vehicle is being subjected to EXTREME service such as towing extremely heavy loads, you may want to consider going to one of the heavier recommended viscosities. 3) If you are not comfortable venturing outside of OEM viscosity recommendations, then, by all means stick with what your vehicle manufacturer recommends. OEM viscosity recommendations will be perfectly safe for your vehicle and will make your selection process easy.
Specialized Cases
It is possible, depending upon how you are using your vehicles or equipment, that a more extreme divergence from the OEM viscosity recommendation might be appropriate. However, if you think this may be the case, before making the decision to diverge entirely from the manufacturer's viscosity recommendation, please do your homework.
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