Synthetic will not cause leaks when warmed up. If you have any leaks, they were there before, and you should have noticed them with conventional oil already.
Some basic oil information:
All Motor Oil will have a certain viscosity designation ... we're all familiar with, eg: 30 or 10W-30. The one or two numbers represent what new, unused oil viscosity characteristics it will have. For simplicity, we will ignore low grade oil, but be forewarned ... it is possible to buy oil that has no additives (eg: API-SA) or is not winter rated but is multigrade. (eg 20-30, not 20W-30). Do not use these oils, period.
There should always be a "donut" label from the American Petroleum Institute on the oil you buy. Read your owner's manual (or, for JDM engines, refer to a similar engine family and year of manufacture) for the minimum grade to look for. It is very important that people realize that you can buy oil today on the shelves that does not meet your engine's minimum requirements.
You should be looking for:
S~ classifications are for gasoline engines; C~ classifications apply to Diesel engines. (If you are wondering why not G and D, they are based on ignition. Gasoline = S = "Spark Ignition; Diesel = C = "Compression Ignition". With any luck, knowing that will help you remember if you have a brain fart at the store).
Other classifications (Europe, Japan) certified by other bodies may be listed; typically International Lubricant Standardization and Approval Committee (ILSAC).
Gasoline Engines:
SN: Recommended for 2011 and newer gasoline engines.
SM: Recommended for 2010 and earlier gasoline engines.
*** IMPORTANT: Special Note about SN, SM and "Energy Saving" API rated oils ****
*** IMPORTANT: Using the wrong oil can harm your Diesel Engine ****
If you own an older gasoline car with a flat tappet camshaft (newer cars generally have roller tappet cams) or have done high performance valve train modifications with high lift cams and/or high valve spring pressures, or you have a Diesel engine, DO NOT USE any oil rated ONLY for gasoline engines and classified as either meeting SN or SM. In some cases SL rated oil is also detrimental to your engine.
To meet EPA and Transport Canada requirements beginning with the 2005 model year that mandated all catalytic converters must be warranted for at least 120,000 miles /180,000 km, the level of zinc phosphate was drastically reduced in these oils. (Also known as Zinc Dialkyl Dithio Phosphate or simply "ZDDP"). ZP is a high pressure lubricant found as part of the additive package in motor oil that does not wash off engine parts, including cam lobes when the engine is shut down.
It is also important for Diesel engines that experience extreme pressure on engine main bearings due to the inherent design of compression ignition engines. On cold start, serious metal-on-metal wear occurs. When used in engine oil on gasoline engines, it tends to reduce the life of catalytic converters and O2 sensors slightly ... not a lot, but enough to prevent meeting the EPA/Transport Canada minimum requirement introduced in 2004.
If the SL oil formula is also low level ZP, it will have the "Energy Conserving" label in the API donut. DO NOT USE ANY "ENERGY CONSERVING" RATED OIL IN THESE ENGINES. Permanent damage requiring engine overhaul has been found in as little as 500 miles of driving.
Engine Oils that are classified as "Heavy Duty" (may appear anywhere on the label) for gasoline engines or are also rated for Diesel engines with classification CI-4 or CI-4+ may be used, as Diesel engines will also fail without sufficient amounts of this oil additive.
By the way, this marks the first time in over 100 years that an API service rating does not apply universally as safe for all previously made engines and the first time using a newer oil formula can cause catastrophic engine failure. Since the EPA has made no secret of it's desire to rid the world of pre-1996 automobiles, it does make you wonder if they cackled with glee knowing that they were creating a regulation that would send these vehicles to the crusher without the owner even realizing how it happened to a perfectly decent running motor.
**** End of Special Note ****
SL: Recommended for 2004 and older model gasoline engines. See 'Special Note' above regarding "Energy Saving" classification in the API donut label above.
SJ: 2001 and earlier gasoline engines.
SH: Obsolete; not recommended alone, but is current for these gasoline engines if also rated for Diesel (C- ...) on the API donut label.
Sometimes you will find multiple designations (eg: SH-SJ). The older is just there to get people who actually read the owner's manual and do exactly what it says, to buy it; the newer one is the actual classification.
All other S~ ... classifications are considered obsolete for gasoline engines; even very old cars and trucks (SA; SB, SC, SD, SE, SF, SG). May be found for sale on store shelves or bulk oil; there is no prohibition on selling this oil. Avoid.
ILSAC ratings:
These are somewhat simpler than API ratings. A new category supersedes the previous, so for example SF-5 replaces SF-4 as soon as the newer standard is introduced. I have no accurate information on how these ratings affect older cars, except that if the oil is marked "Resource Conserving" in the API label it means it also meets SF-5.
SF-5: Current. Introduced 2011.
SF-4: Obsolete. Use SF-5
SF-3: Obsolete. Use SF-5
SF-2: Obsolete. Use SF-5
SF-2: Obsolete. Use SF-5
SF-1: Obsolete. Use SF-5
---------------------
Diesel Engines:
If an oil is Diesel rated as well as Gasoline rated, the Diesel performance requirements supersede the Gasoline performance requirements. Therefore a dual-rated oil will be suitable for cars that should not use the low zinc phosphate gasoline-only rated oils referred to above in the 'Special Note".
CJ-4 Recommended for 2010 and newer Diesel engines.
CI-4 /CI-4+ Introduced for 2004 model year Diesel engines.
CH-4 Introduced for 1998 model year Diesel engines.
CG-4: Obsolete. Use CH-4, CH-4+, CJ-4 rated oils.
CD, CD-II, CE, CF, CF-2, CF-4 ... obsolete. Use CH-4, CH-4+, CJ-4 rated oils.
CA, CB, CC ... Obsolete. WARNING: Do not use.
Examples of obsolete oil found for sale on store shelves:
POQI: Obsolete Grades and Warnings:
http://www.pqiamerica.com/Labels.htm