I2C Pullup resistor question

Hi,
I have a 3.3V I2C sensor and I want to connect it to my Arduino Mega (which has a 5V SDA & SCL voltage);

There is one thing I don't understand:
The Arduino has two internal pullup-resistors to set the two lines (SDA, SCL) to 5V. When I want to connect my 3.3V sensor to the Arduino people tell me that I need a level shifter. But why can't I just disable the internal pull-up resistors and add external pull-up resistors to it, but with 3.3V supply?
Then the HIGH voltage on SDA would be 3,3V and as far as I know, the Arduino recognizes something as HIGH when it's 0.6 * Vcc (0.6 * 5 = 3V) which is under 3.3V. (or are this 0.3V too small to be safe?)

Can someone explain to me why this can't work because I can't figure it out. Thanks!

The 3V3 and 5V logic are incompatible. You can destroy weaker chip so you have to use level shifter.
Pull-up resistors have different function (like for open collector switching) in case of using both voltage levels, the pull-up have to be placed in both bus parts - on the 3V3 side and also on the 5V side.
Internal pull-up is not feasible. The resistance is too high.

Budvar10:
The 3V3 and 5V logic are incompatible. You can destroy weaker chip so you have to use level shifter.
Pull-up resistors have different function (like for open collector switching) in case of using both voltage levels, the pull-up have to be placed in both bus parts - on the 3V3 side and also on the 5V side.
Internal pull-up is not feasible. The resistance is too high.

Thanks!
Can i still use the internal pull-ups on the 5V side or should i switch to external ones?

An external, definitely. The internal one is ~20-30k but feasible value should be around 5k. Even the value is not critical it should be something between 1k2 and 10k in dependance on situation as bus length, interference etc.
Usually 4k7 value is used for pull-up resistor. It is good for the most of cases.

Hi,

i to had the same issue the easiest solution is to buy one of the many level shifters you can buy on ebay and similar sites, these have 10k pullup resistors for the low and high sides

see examples below