I reckon it is also not a good idea to let the mechanic open up this old engine to check here & there... I worried later need to end up doing a full-overhaul. (Because now the engine is still running perfectly fine. )
Oh my, I've never been checking my engine oil level at all... (I'll keep it in mind to occasionally check from now on...)
So far, this old engine does not have any oil leaking problems at all, but I reckon I should start the practice of checking the oil level from time to time...

Will the mechanic have to open up the whole engine to check the "oil seal & gasket"? (Maybe this two components is something I can tell the mechanic to check/replace if necessary...)
But the issue with the 10W - 30 engine-oil grade is that it won't offer so much protection (as the 10W - 40 grade) when the engine (and the weather) gets hotter isn't it? (It will break down faster under higher-temperature & possibly cause sludge build-up isn't it?)
But how come the mechanic (last time) advice me not to go down to 5W - 40 (a thinner oil grade) from my current 15W - 40 (a thicker oil grade), because he told me that the engine oil will start leaking (because it will be too "thin" for my old engine to handle...)

Hence, I don't think the 10W - 40 engine-oil is having the "same" viscosity-level as the 15W - 40 engine-oil (which will naturally still be the thicker oil)...

Correct me if I got it wrong...
Again, I still think that the 10W - 40 engine-oil grade is "thinner" than the 15W - 40 grade. (However, both engine-oil grades will be having the same "resilience-level" at high-temperatures.)

Again, I am worried that the 10W - 30 engine-oil grade will not be as good as the 10W - 40 engine-oil grade in protecting my engine at higher-temperatures (hotter weather & longer idling in traffic-jams etc... as such.) Because it will break down faster than the 10W - 40 grade engine-oil... (I think...)

Now assuming there is a 10W - 60 engine-oil grade, it's viscosity-level will still be exactly the same as the 10W - 40 engine-oil grade, but the 10W - 60 engine-oil grade will be having a significantly higher/better "resilience-level" at higher-temperatures (it will take longer for it to break-down under higher-temperatures - the oil will have a significantly better chance to last longer).

Correct me if I'm wrong. (I hope I am not confused...)

Thanks a lot to you guys BTW.

Just out of curiosity, what will happen if I put the 0W - 50 engine-oil grade into my old engine? (According to articles, a 0W grade oil will pour like water, so I reckon my old engine will be able to take it, as it will be far too thin/watery... )

But are you saying that the "viscosity" of the engine-oil is determined not by the xW (0W/5W/10W/15W/20W) number, but instead, by the number after it (like the 30/40/50)?
BTW (I always noticed);

That 20W - 50 is always a (cheaper) mineral based engine-oil.

But 0W - 50 is always a (more expensive) "fully-synthetic" based engine-oil.

Why is that so?

I'm not sure, but all of this is really quite confusing to me...

But generally, I always noticed that 20W - 50 is always the (cheaper) "mineral" based engine-oil that is always mentioned to be (too thick) and highly-recommended to be used in older car engines...

0W - 50 on the other hand is always the (more expensive) "fully-synthetic" engine-oil that is only recommended to be used in brand-new car engines... (because it is too thin to be used in older car engines...)

How true is that?

In addition, I have also never ever came across a 20W - 50 engine-oil that is "fully-synthetic" before... (Why isn't there any out there?)
And I have also never ever came across a 0W - 50 " engine-oil that is "mineral" based before... (Why is that so?)

I really don't know, but going for a lower second number such as the xW - 30 (like you said) will mean that my engine will have lesser protection at higher-temperatures..., as well as the engine-oil will also "break-down" faster and hence; need to be replaced faster (because of lower endurance/resilience to heat)... (Isn't it?)

But based on your information/theory; the 0W - 50 grade will be one heck of a super "thick" engine-oil isn't it? (I thought that the racing industry already no longer uses thick/super-thick engine-oils for their race vehicle's engines anymore... ) - I was informed that now a days all racing vehicles are using much thinner fully-synthetic based engine-oils (for a higher-performance), such as the 0W - 50/0W - 60 grade, that comes with the same high resistance/endurance to heat & sheer at high temperatures...

Nvm, I would put aside my doubts for now. 

It would be better to return back to the main topic for now (although further discussions on engine-oils will always be more than welcomed.);
So which engine-oil grade would you experienced guys in here recommend me to put inside my old engine for this coming service?

Currently I'm using the 15W - 40 Mineral-based.

But if I would like to have a higher engine performance, easier & less-damaging cold starts (without the need to warm up the engine for so long), and better fuel-efficiency; which engine-oil grade would you guys recommend me to use?

Is it xW - 30? (Or any other grades?)

Should I go for fully-synthetic?

I'll really appreciate your help/advice.

Sorry, I really have no idea what oil viscosity-rating does my car engine specs states, however, this engine has always been using the 20W - 50 mineral-based oil in the past.

There have not been any blue/white/other color smoke coming out from the exhaust-pipe so far.
I am still having the Shell 10W - 40 semi-synthetic oil in mind; also because of it's engine-cleansing quality advertised. (However, I am opened to your recommendations.)

But if I ever see a xW - 30 oil-grade from Shell, then I will be going for that instead of the above... (Because like you guys had said, the "30" denotes a thinner oil grade than "40" which will increase engine performance & fuel-efficiency...)
The other two engine-oil grades I have always been seeing from Shell (in stores & workshops) is the 15W - 40 Mineral (which I'm using currently), and the 5W - 40 Fully-Synthetic (which could be pointless for this old engine like you said...). Hence, I will select the 10W - 40 Semi-Synthetic from Shell for this next oil change, but it would be good if you can also leave some of your recommendations in here, if you have any.

Quazacolt, thanks a lot.

However, I'm still in doubt as to whether the 0W - 50 engine-oil (from Mobil for example); is really a very "thin" or a "very-thick" engine-oil? (Because the way Mobil promotes & advertises it, is as if this oil is a very ultra "thin" super-grade engine-oil that can still 100% protect the engine and can also "withstand super high-temperatures" at the same time; which is very ideal for the high-performance/racing type engines as they say... ) They also mentioned that this 0W - 50 performance oil provides maximum fuel-efficiency... (Which seems to  denote an engine-oil that is "thin"...)

But how is the above so-called "super-oil" different from the (much cheaper) 20W - 50 engine-oil grade (that I've always been using)? (Since in Malaysia's climate, the xW number in the front does not matters...?)

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Added on February 3, 2012, 6:16 pm

Ford-Laser 1600cc (Carburetor) Manual 5-Speed Transmission. (A very durable engine with strong-torque.)

But I'm getting quite bored with this car now, so I have plans to go for a different car soon.

OK, thanks bro for dispatching the "correct" information.

I also reckon that I have not made a mistake by going for this current xW - 40 (thinner) engine-oil grade (from my previous 20W - 50 grade) in the last oil change.

I also change engine-oil "rather frequently" BTW; at every three months. (Because I can't change oil by referring to my mileage meter because it goes up extremely quickly; in just one month, the meter would already have moved up to around 5000+ kilometers; when I have hardly even drove out the car to begin with...; Nobody uses this car except for me, and I always lock-up the only set of keys. )

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Added on February 3, 2012, 6:46 pm

Thanks for the information & advice.

Yeah, it's true, all the mechanics always recommend the 20W - 50 engine-oil to be used for this old car's engine, but I don't think that this car's engine even need such a thick engine-oil grade... (It does not have oil leaking & white/blue smoke issue problems...)

OK, thanks man.

The very "only" times that white smoke ever does shoot out from my exhaust pipe is when I have sprayed the Carburetor-Cleaner solution into the carburetor's air-intake to clean it up nicely. (I do it once a week whenever I have drove the car out for at least 5 times, otherwise it will be done just once every two weeks...) The white-smoke will then completely disappear after a while.

However, whenever the weather get very cool at-night after a heavy-rain, some white-smoke usually does come out from the exhaust-pipe; but it usually stopped once the engine has fully warmed-up... (I usually wonder what caused it; although I have read that it is normally caused by water-vapors accumulated inside the exhaust pipe.)

BTW, for engine-revving above 3K to around 4K; which engine oil (type/grade) is generally suitable? (Because I always have the fear of revving my engine above 3K due to the fact that I do not think that my Mineral-based engine oil will be resilient enough to handle it...)

Wow, you're just like my bro haha... (He also revs his engine past 5K RPM quite often, and he is using 10W - 40 Semi-Synthetic engine oil...)

However, revving the engine is one thing (because it is easy to do it), but whether the engine-oil can handle it or not is another story... (It may break down faster & stop protecting your engine before the next oil change interval...) Which is what I fear...

I would like to rev my engine at-least to 3K+ RPM and still have the confidence that my engine-oil can take it, and it will not break down before time...

For this case, is Semi-Synthetic recommended over Mineral-based oil? (Would you recommend the xW - 30 grade, or is the xW - 40 grade going to offer more protection?)

Thanks.

Interesting...

But there is a saying/tactic (I heard) that using a "cheaper" engine-oil & changing it more frequently (offers your engine a better protection); compared to using a "more expensive" engine oil & leaving it running inside your engine for an extended period of time... (This is because the addictive(s) inside the engine-oil will all break down at the same time regardless of being a Mineral or Fully-Synthetic oil grade...) Which means that the engine-cleansing addictive(s) inside a "fully-synthetic" Shell engine-oil (for example), will break down "earlier" than the engine-oil itself; so after the addictive(s) had "broken-down", the engine-oil will no longer have it's cleaning-properties anymore... (I wonder how true is this...)

Provided the above is really true, then I reckon it would just be a better idea for me to stick to a Mineral grade engine-oil isn't it? (With the exact same engine-cleaning properties) and just change it more frequently that's all (instead of using a more expensive semi-synthetic or fully-synthetic grade).

This is just my little concern/doubt left about going for a more-expensive engine-oil and letting it run inside the engine for longer... (And can the oil-filter also last as long?)
Anyway, if a semi-synthetic engine-oil can indeed provide me with a better protection during high engine revving (3K+ RPM), and will take longer to "deteriorate" (under constant use); then I will still go for it regardless...

BTW, where are those liqui moly/torco/amsoil/redline/motul engines-oils being sold? (I've never came across any of those oil brands in real-life before...)

Thanks a lot for all your information BTW.

blue and white smoke indicate something about ur engine combustion......

i'm not pretty sure bout car engine and i forget what do white smoke indicate but blue smoke cause by an excessive lubricant during engine combustion

whether its cause by thicker or thinner lubricant i'm not sure

u r telling me i'm wrong or giving me the answer?

anyway thanks 4 the link...