Mineral oil and hydraulic clutch slave cylinders.

You might be here because you've wondered, "Can I convert my older hydraulic clutch to mineral oil?" Starting with the R1200GS and K1200S, BMW started using mineral oil instead of brake fluid in the clutch system. It does not require changing (ever) and of course is non-corrosive if it happens to get on the paint.

The test

Interestingly enough, one was already swollen a bit larger than the other, probably due to gear oil contamination. That slave cylinder was full of sludge and the throwout bearing was frozen. Since we want to witness the change due to mineral oil, I used the non-swollen seal for the mineral oil and the slightly swollen one for the DOT 4.

Both seals were cleaned using hot water and Marine Clean, which is a sort of super-Windex sold by the people who sell POR-15. I didn't want to use anything solvent-based.

Results

	Seal #1 Vitamol	Seal #2 DOT 4	Comments
Initial diameter	0.975"	1.022"	Seal #2 larger, likely due to previous contamination
After 6 hours	1.020"	1.022"	Same size now.
After 24 hours	1.055"	1.022"
After 50 hours	1.090"	1.023"	I suspect seal #1 would be sticking in the bore by now.
After one week	1.190"	1.024"	Seal #1 diameter increased by 20%.
After two weeks	1.210"	1.025"	No sign of material deterioration, though.
After three weeks	1.210"	1.028"
After 2 months	1.210"	1.032"	Still no material deterioration.
After 4 months	1.210"	1.030"
18 months	1.210"	1.043"

 

Preliminary conclusion

It doesn't work, and the seals will be permanently damaged by the mineral oil within hours.