Since M1 5w40 TSUV isn't available I switched to RTS 5w40. I still have a quart of M1 left. I've never mixed oils before, but didn't want to waste the M1 - is there any problem if I use the M1 for top ups even though I used RTS for my last oil change?
What is RTS?
If it's Rotella, then you mixing add-packs from two different companies - which can shorten base oil life & additive life because one company may have an additive the other one doesn't -- as a result, that additive only becomes one-half effective when mixed with another base oil.
Sounds complicated?... well the way I worded it even has me confused.
Go ahead, mix it, you'll never know the difference. I run RTS in my 89 turbo Caravan. When I change oil, instead of measuring out a fifth quart of RTS, I use whatever is available, sometimes 15/40 rotella, or ST, or 10/30 Formula Shell. I think on the next change I'll try M1 5/30. Maybe add a shot of VSOT and run it for 2 years.