Difference in use of “also” and “deshalb”

Question

Is there any difference in use and meaning of also and deshalb?

They look the same to me. Can we use them interchangeably?

Also finde ich das Auto gut.

Deshalb finde ich das Auto gut.

Answer 1

While also and deshalb are often interchangeable, fundamentally, there is a difference. Namely, also originally indicates conclusion, whereas deshalb indicates causality. Hence, we could list analogues as follows:

also : thus, hence (and interjections, such as "well")

deshalb: therefore, because of that,...

Consider the following examples:

Er trägt das rote Wappen, also ist er ein Lancaster. -- He wares the crimson coat of arms, hence he is a Lancaster.

the red coat has nothing to do with the reason he is a Lancaster.

Er wurde in Lancashire als Leibeigner geboren, deshalb dient er den Lancasters-- He was born a bondservant in Lancashire, therefore he serves the Lancasters.

A causal relationship can usually be framed as a conclusion, hence deshalb can usually be replaced by also, while the opposite is less commonly true and often a (small and common) act of sloppiness.

However, the sentence order:

Also finde ich das Auto gut.

is valid only for conclusions and in the order:

Also, ich finde das Auto gut.

also is always an interjection.

Answer 2

In this example, the two are indeed interchangeable. Both express a relation of cause and effect, with 'deshalb' putting a bit more emphasis on the 'cause-and-effect' part.

The 'Also' sentence sounds a tad more informal and weaker, so I would hesitate to use 'also' in a mathematical proof where only very strict causation is expressed (either it does follow, or it does not follow -- no medium allowed). However, for anything not as strict as a written mathematical proof or a formal law text, you can use them interchangeably in this meaning.

However, while 'deshalb' can only be used for cause and effect, 'also' can also have different uses (dict.leo.org)

As a colloquial filler:

Also, das habe ich nicht gewusst!

Wow / Really / Well, I didn't know that!

Sie gibt mir also 30 Cent und ich soll davon ein Mittagessen kaufen.

So she gives me like 30 cents and I am supposed to buy lunch from that.

Or as emphasis:

Er hat also doch die Schokolade gegessen.

He did eat the chocolate after all.