When I do an oil change, I always buy 5 quarts of motor oil... I always have 1/2 quart remaining that I never use after... So, what I wan't to know is, do the remaining oil in the quart lose some of it's properties over time since it's been opened for a while (but never put in the engine)? Some of my quarts are over 1 year old... Are they still as good as new oil?

They are as good as the oil was when you got it, as long as the the container is kept closed.

Since formulations and ratings change all the time, whether it is is as good as "new" oil is open to debate.

Yes they were closed... It's normal 5w30 ACDelco oil, the new quarts I buy are the same numbers and all... Should be okay to mix the old and the new together then?

I will drag race (bracket racing) my car 3 times in the whole summer, my mechanic say I should absolutely put 5w30 full synthetic oil... But I don't know, from what I have read here, it could make my engine lose more oil after that because the oil infiltrate everywhere. I am right? Or should I buy full synthetic oil like he said? If yes, what brand?

My car is a 88 GTA, 305 TPI, automatic by the way with 117 000 original miles with absolutely no mods...

As long as you use the oldest left-over oil first, there shouldn't be a problem mixing it with the newly purchased oil.

Synthetic is a generally better lubricant. For intermittent use/long storage, a high TBN is a good thing to have, which you'll get in a quality synthetic. If you go synthetic, don't waste you money on the pretend stuff - which means most of the major oil companies and store brands. You'll be okay with Mobil 1, AMSOIL, Redline, or Royal Purple, which are PAO based products (although AMSOIL and I believe Mobil 1 also have the lower grade stuff available).

If I buy the cheapest synthetic oil, will it still be better than a good brand 5w30? Let say I buy Tech2000 synthetic (Wal-Mart brand in Canada I think), will I be okay with this?

When I use regular 5w30 oil, my car doesn't drop or use any oil in the 3000 miles interval, so should I buy 5w30 for synthetic too?

If you buy 5 litres of oil every time and have 1/2 litre left over, why not just buy 4 litres on every second oil change and use the 1/2 litre that was left over from the previous change?

don't our cars require 5.5 quarts?

4 plus whatever the filter holds.

The cheap synthetics are typically a little better than regular petroleum oil.

did the older cars use a smaller pan? because the number 5.5 is VERY VERY VERY familiar when talking oil....

My car takes 5.5, but I use a larger filter and have an oil cooler with all its associated plumbing. All of that easily provides for an extra litre over stock.

So I can buy Tech 2000 (Wal-Mart Canada) 100% synthetic oil without worries with my engine that have 117 000 original miles? Will it be better than any regular oil brand?

The filter, ACDelco or something else?

The only concern typically is hardened valve stem seals, which may suck down a little more oil than you'd like. They aren't that hard to replace. But, the same thing is happening with petroleum oil, you just don't care enough.

A quality oil filter. Pick your brand. I'm biased towards AMSOIL products, and most anyone who's honest will admit they have a bias towards a particular brand that isn't exactly based on scientific data (except me, of course...).