Derek, first off, there usually is no problem encountered when mixing synthetic and petroleum-based (more commonly know as conventional) oils.
Conventional mineral motor oils are refined using crude oil for the base stock which contain impurities, such as nitrogen, wax, carbon, sulfur and aromatic residues. Refining removes most impurities but some remain to cause problems, especially under high stress and temperatures.
Synthetic oils are developed in the laboratory from man-made organic esters and other synthesized hydrocarbons. These oils contain no impurities, but impurities can appear during combustion.
Even the best mineral-based motor oils break down or oxidize at temperatures of 121°C to 149°C (250° to 300°F). At temperatures above this the life of these oils drops drastically, resulting in increased engine wear and formation of harmful sludge, varnish and other deposits. Synthetic oils can easily tolerate temperatures of up to 232°C (450°F), and some can go up to 371°C (700°F). Synthetic oils also resist oxidation up to 10 times longer.
Synthetic oils work better at low temperatures because they flow much more freely, down to -40°C to -46°C (-40° to -43°F). Synthetic oils also start lubricating sooner and reach the valve train faster.
Because synthetic oils are slipperier, friction is reduced, but fuel economy is improved only a little; the real economic payoff comes from fewer repairs.
Long intervals between oil changes are possible because synthetics resist oxidation, suffer less viscosity breakdown and decompose less into sludge. But don't overdo it.
Filters should be changed at the normal recommended intervals.
Be careful about using synthetics in an engine with high mileage: the new oil could include a different detergent that can break loose accumulated deposits, clog oil passages and cause other havoc inside the engine.
Secondly, be careful about having too low of an oil temperature. You need to get the oil warm enough to vaporize any contaminents that find their way into the oiling system. You should expect to keep the oil temperature around 93°C (200°F) in order to be effective. The oil temperature won't even give you a read-out until you hit approximately 65°C (150°F), and you don't out of range until you go above 149°C (300°F).
Finally, as for gasoline additives, it doesn't hurt to run a bottle of injector cleaner through a tank of gas every so often, but if you are always using a quality gasoline from a reputable dealer, there usually is no need for additional additives. The additives vary from each oil company, but the gasoline often is the same no matter what label you may see on the pump. This happens because of what is known as "common carrier" pipelines; a barrel of gasoline that goes nto that pipeline in Texas is not going to be the same barrel that comes out of the pipeline somewhere on the east coast.
I hope I didn't muddy the waters any more than necessary. Actually, I hope it helped you.
_ken
4LT GS Roadster
