as for the 20 days interest free, it goes like this

1. Every month the bank wil on a specific date (called Statement Date) issue you a statement informing you of you account status. And usually you need to make payment before the 20th day (Due Date) after this said Statement Date. For example:

Your Statement Date is on the 8th of every month. Therefore payment is due on or before 28th of that particular month. No interest will be imposed if you make payment before or on the Due Date. That's why they say 20 days interest free. 

Actually you can enjoy more than 20 days interest free. Once again will take 8th of  every Janaury 2011 to be your Statement Date. Now you go make a purchase on the 10th of January which is pass your Statement Date and this transaction amount will only be shown in the 8th of February 2011 Statement Date and payment due on 28th February 2011. That means you actually got yourself more than 40 days interest free on your purchase if you pay in full on or before 28th Feb 2011.

To read more on credit card, click on my signature below - Intro to CC101

but like Gen-X mentioned, its actually more than 20 days.
let say i swipe rm3,000 on 29/12/2001, it will be billed on 28/01/2011, and i need to pay it on 17/02/2011.
if calculated exactly, its 51 days~

ur welcome ~

installment plan amount will be included in the statement balance.
as long as u pay full amount of statement balance by due date, ur installment balance wont be charged with interest

ur welcome.

if ur annual salary is rm36k, go for OCBC Titanium. 5% rebate at all petrol station.
the best card for petrol rebate so far.

regarding installment, u want do it at merchant or flexi payment?
flexi payment is convert retail purchase (full amount swipe) into installment (call/sms bank later).
this is useful for merchant that dont offer installment plan.

cimb petronas only give rebate at petronas. useful if u pump with them only.
their flexi payment is free for 10 months on hospital, insurance and education.
others, 2% for 10 months or 4% for 20 months.

ambank also offer flexi payment free for 6 months and 12 months.
their petrol rebate is 2% at all petrol station (ambg carz visa/mc).
min requirement is rm24k/year.

pbb also offer flexi payment free for 6 months or 1.5% for 12 months (all transaction).
but their petrol rebate is at esso/mobil only (pbb esso/mobil visa), between 0.5% to 5% (tier-based).
the min req is rm18k/year.

u choose urself