Arduino and SQ-1

Background:

Korg SQ-1 is a sequencer for analog synthesizers.
It provides a way to sync rhythm from an external clock.
It has a SYNC IN and a SYNC OUT port in the form of a female mono jack 3.5mm.
According to SQ-1 manual, when an advance in time must occur, SYNC OUT port outputs a 5v pulse 15ms long.
When an external clock is used, SYNC IN interpret as an advance in time a pulse like this one, but the manual also specifies that SYNC IN maximmum input level is 20v.

Other relevant things:

Even if the manual says that SYNC OUT outputs a 5v pulse, that's not true. I measured it with a multimeter, an I saw that the maximum voltage was more or less 0.3v.
I connected an Arduino digital pin to SYNC IN through a voltage divider, in this way I discovered that a pulse of 0.46v is correctly interpreted as an advance in time.
I also directly connected an Arduino digital pin to SYNC IN, and saw that the sequencer worked fine with direct pulses of 5v.

My problem:

I want to use an Arduino as a clock generator to drive this and other similar sequencers at the same time.
I like the idea of directly connecting the Arduino pins to the sequencer, because this approach saves time and components. But I am not sure if something like this is safe in the long run.

Should I add components to make some kind of protection?

The Korg manual says nothing about input or output current. Should I be worried about the amout of amperes Arduino is going to supply to the sequencer?

In order to protect Arduino and/or SQ-1, should I at least wire a resistor between Arduino and SYNC IN?

Regards

Even if the manual says that SYNC OUT outputs a 5v pulse, that's not true. I measured it with a multimeter, an I saw that the maximum voltage was more or less 0.3v.

You can not measure a pulse with a multimeter, it just will not work.

I connected an Arduino digital pin to SYNC IN through a voltage divider, in this way I discovered that a pulse of 0.46v is correctly interpreted as an advance in time.

No feed the 5V directly into it to get better noise immunity.

But I am not sure if something like this is safe in the long run.

It should be from what you have said so far.

Should I be worried about the amout of amperes Arduino is going to supply to the sequencer?

Have a look at the circuit, does the input go into a black IC? If so it should be safe. Even so the load should not be excessive.

In order to protect Arduino and/or SQ-1, should I at least wire a resistor between Arduino and SYNC IN?

Well if it were me then I wouldn't bother. Don't forget to connect the grounds of the two systems together.